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Solid lubricant coatings of WS 2 and Cr–WS 2 (15–50 at.% Cr) prepared using an unbalanced magnetron sputtering system were evaluated for their mechanical and tribological properties. Nanoindentation results indicated that addition of Cr helped in improving the mechanical properties and the elastic recovery ability of Cr–WS 2 coatings. The adhesive strengths of Cr–WS 2 coatings were evaluated using a nanoscratch tester and from the nanoscratch profiles, critical load values and optical images, it was evident that the adhesion of Cr–WS 2 coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS 2 coatings exhibited large amount of plastic deformation compared to Cr–WS 2 coatings which showed a combination of elastic–plastic deformation. However, micro-tribometer measurements at a load of 2 N showed that the tribological properties of Cr–WS 2 coatings deteriorated with an increase in the Cr content. For example, Cr–WS 2 coatings prepared at Cr content ≥ 33 at.% failed after a sliding distance of 1 m. On the other hand, WS 2 and Cr–WS 2 coatings prepared at low Cr contents (15–23 at.% Cr) exhibited a stable friction coefficient (50–60% relative humidity) in the range of 0.10–0.13 for a sliding distance of 14 m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14 m indicated the presence of WS 2 transfer films for WS 2 and Cr–WS 2 coatings prepared at low Cr contents. For Cr–WS 2 coatings with Cr content ≥ 33 at.%, the worn films consisted predominantly of WO 3 . After an extended sliding distance of 50 m, Cr–WS 2 coatings (15–23 at.% Cr) outperformed WS 2 coating which failed after 20 m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200 m. At a higher load of 7 N, Cr–WS 2 coating with 15 at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72 m. These results indicate that the amount of Cr in the WS 2 matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr–WS 2 solid lubricant coatings.

WS2 and Cr–WS2 nanocomposite coatings were deposited at different Cr contents (approximately 15–50 at. %) on silicon and mild steel substrates using an unbalanced magnetron sputtering system. X-ray diffraction (XRD) was used to study the structure of Cr–WS2 coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr–WS2 coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO3 on the surface of Cr–WS2 coatings and the intensity of the oxide phase decreased with an increase in the Cr content, which was also confirmed using energy-dispersive X-ray analysis results. The surface morphologies of WS2 and Cr–WS2 coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS2 strongly influences the structure and morphology of Cr–WS2 coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr–WS2 coatings resulted in a structural transition from a mixture of nanocrystalline and amorphous phases to a complete amorphous phase. The cross-sectional FESEM data of WS2 coating showed a porous and columnar microstructure. For the Cr–WS2 coatings, a mixture of columnar and featureless microstructure was observed at low Cr ontents (≤23 at.%),whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr–WS2 coatings prepared at Cr content ≤23 at.% indicated the presence of both nanocrystalline (near the interface) and amorphous phases (near the surface). Furthermore, high-resolution TEM data obtained from the nanocrystalline region showed inclusion of traces of amorphous phase in the nanocrystalline WS2 phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr–WS2 coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.