Structural, mechanical and tribological properties evolution in N-incorporated CrS films

In this paper, series of N-doped Cr-S-N films, for the first time, have been synthesized by magnetron sputtering CrS target in the mixed atmosphere of N-2 and Ar. It is found that with the N-2 flow increasing from 0 to 30 sccm, a continuous increase in N content (from 0 to 31.2 at.%) occurs. Structurally, the N introduction leads to a phase transition from hexagonal CrS with poor crystallinity to completely amorphous Cr-S-N, as evidenced by the combination of XPS, XRD and TEM results. Followed by a creep cross-sectional observation changing to a columnar growth, and finally to a flat and dense microstructure with increasing N content. On the other hand, the mechanical and tribological properties are found to be strongly dependent on N content (structural variation). To be specific, when the N content increases from 0 to 26.4 at.%, the hardness gradually increases from 7.4 +/- 0.4 GPa to a maximum value at 15.3 +/- 0.3 GPa, and slightly decreases at higher N content (31.2 at.%). This favorable tendency for hardness suggests that N incorporation into CrS film is an effective approach for significantly improved hardness (two times higher). Better yet, such N content also enhance the fracture toughness, lower the friction coefficient and wear rate. The enhanced hardness and wear resistance can be attributed to the synergistic effects from hard CrNx phase, dense microstructure and compressive stress, while the low friction can be ascribed to the formation of CrSx lubricant at sliding interface.