DCMS Mo-Se-C solid lubricant coatings – Synthesis, structural, mechanical and tribological property investigation

Abstract

Mo-Se-C is a very less explored subset of carbon-based transition metal dichalcogenide (TMD-C) nanocomposite coatings but a potential candidate as solid lubricant coatings in aerospace and automotive industries due to excellent frictional stability both in humid air and vacuum conditions. The present work aims at the development and investigation of dc magnetron co-sputtered Mo-Se-C coatings using separate C and MoSe2 targets, as a first step for industrial up-scaling. By varying the applied C power, a carbon content of 44 to 60 at. % was achieved. 50 at. % C was selected for the deposition of coatings with negative substrate bias of 50, 70 and 90 V dc voltage and, for the latter, pulsed dc bias was also applied. Maximum Se/Mo ratio of 1.88 was evaluated by wavelength dispersive spectroscopy (WDS) analysis. After the application of substrate bias, the deposition rate and Se/Mo ratio decreased while higher compactness was observed by scanning electron microscopy (SEM). Mo-Se-C coatings displayed broad amorphous grazing incidence X-ray diffraction patterns. Transmission electron microscopy (TEM) showed MoSe2 platelets randomly oriented in amorphous carbon matrix. X-ray photoelectron spectroscopy (XPS) was not conclusive towards the presence of Mo-C bonds while Raman analysis showed the presence of MoSe2 crystalline peaks. Nano-indentation tests showed an increase in the hardness of coatings deposited under negative substrate bias. Tribological tests were performed in humid air and dry nitrogen. 400C(90V) coating displayed the highest frictional stability in both environments, while the lowest friction coefficient of 0.025 and specific wear rate of 2.4 × 10-8 mm3/Nm were achieved for tests in dry nitrogen.