Structure, morphology and chemical composition of sputter deposited nanostructured Cr–WS 2 solid lubricant coatings

Abstract

WS 2 and Cr–WS 2 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–WS 2 coatings and the bonding structure of the coatings was studied using X-ray photoelectron spectroscopy (XPS). The characterization of different phases present in Cr–WS 2 coatings was carried out using micro-Raman spectroscopy. The XPS and Raman data indicated the formation of a thin layer of WO 3 on the surface of Cr–WS 2 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 WS 2 and Cr–WS 2 coatings were examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy. It has been demonstrated that incorporation of Cr in WS 2 strongly influences the structure and morphology of Cr–WS 2 coatings. The XRD and FESEM results suggested that increase in the Cr content of Cr–WS 2 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 WS 2 coating showed a porous and columnar microstructure. For the Cr–WS 2 coatings, a mixture of columnar and featureless microstructure was observed at low Cr contents (≤ 23 at.%), whereas, a dense and featureless microstructure was observed at high Cr contents. Detailed cross-sectional transmission electron microscopy (TEM) studies of Cr–WS 2 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 WS 2 phase. Potentiodynamic polarization measurements indicated that the corrosion resistance of Cr–WS 2 coatings was superior to that of the uncoated mild steel substrate and the corrosion rate decreased with an increase in the Cr content.