Mechanical and tribological properties of sputter deposited nanostructured Cr–WS2 solid lubricant coatings

Solid lubricant coatings of WS2 and Cr–WS2 (15–50at.% 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–WS2 coatings. The adhesive strengths of Cr–WS2 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–WS2 coatings increased with an increase in the Cr content. Further analysis of the nanoscratch data indicated that WS2 coatings exhibited large amount of plastic deformation compared to Cr–WS2 coatings which showed a combination of elastic–plastic deformation. However, micro-tribometer measurements at a load of 2N showed that the tribological properties of Cr–WS2 coatings deteriorated with an increase in the Cr content. For example, Cr–WS2 coatings prepared at Cr content ≥33at.% failed after a sliding distance of 1m. On the other hand, WS2 and Cr–WS2 coatings prepared at low Cr contents (15–23at.% Cr) exhibited a stable friction coefficient (50–60% relative humidity) in the range of 0.10–0.13 for a sliding distance of 14m. Micro-Raman spectroscopy data of the worn films taken after a sliding distance of 14m indicated the presence of WS2 transfer films for WS2 and Cr–WS2 coatings prepared at low Cr contents. For Cr–WS2 coatings with Cr content ≥33at.%, the worn films consisted predominantly of WO3. After an extended sliding distance of 50m, Cr–WS2 coatings (15–23at.% Cr) outperformed WS2 coating which failed after 20m. Further, the coatings prepared at low Cr contents did not show any failure even after a sliding distance of 200m. At a higher load of 7N, Cr–WS2 coating with 15at.% Cr exhibited the best performance with a friction coefficient of 0.07 up to a sliding distance of 72m. These results indicate that the amount of Cr in the WS2 matrix needs to be controlled judiciously to obtain improved mechanical and tribological properties in Cr–WS2 solid lubricant coatings.