Structure characterization of pulsed laser deposited MoS x –WSe y composite films of tribological interests

Abstract

Pulsed laser deposition (PLD) was employed to grow MoS x –WSe y composite films, where x = 1.18, y = 0.78. Scanning electron micrographs show that the films have a dense granular morphology. Crystallization, d-spacing and hexagonal sheet curvature within the film were studied with X-ray diffraction, electron diffraction and transmission electron microscopy. A predominant hexagonal MoS x phase was formed but contained W and Se, which were most likely present as substituents for Mo and S. There was no evidence for two separate crystalline phases. MoS x –WSe y composite films exhibited a larger expansion along the c-axis (d-spacing between basal planes) than PLD MoS2 and WSe2 films grown by laser ablation of pure targets. The lattice spacing along the a-axis was expanded in comparison to the MoS2 film, and compressed in comparison to the WSe2 film. X-ray photoelectron spectroscopy showed a significant sulfur deficiency, and verified both of S and Se bonding in the film. High-resolution electron microscope images exhibited significant curvatures of the (002) basal planes in the films. The bending behavior of basal planes was explained by S vacancies and Se substitution on the atomic site of S layers. The tribological properties of the composite films were measured in dry and wet conditions using a ball-on-disc tribometer. The reduced friction was correlated with the increased crystallinity and increased separation of basal planes in the composite films.