Control of molybdenum disulfide basal plane orientation during coating growth in pulsed magnetron sputtering discharges

Molybdenum disulfide coatings were grown under selected mid-frequency bi-polar pulsed direct current power conditions to control the basal plane orientation relative to the substrate, from parallel (002) to perpendicular (100). Growth mechanisms under different conditions were made apparent by the observed trends. The highest (002)/(100) orientation ratios were obtained with growth rates near 1 atomic layer per second accompanied by high energy ion bombardment (up to 175 eV). These conditions allowed maximum surface diffusion of deposited atoms before burial by incident material or removal by desorption and resputtering. Additionally, high energy ion bombardment during pulsed deposition produced defects on the otherwise inert MoS2 (002) surface that further enabled adsorption and crystal growth. For growth rates of less than one monolayer per second coupled with low ion energies and fluxes, coatings exhibited strong (100) orientation. Under these low growth rate conditions, the burial rate was still higher than the desorption rate on the (100) surface, and the lack of energetic ion bombardment permitted uninterrupted growth of crystals in this orientation. For sliding wear tests conducted in humid air, the tribological performance of the coatings demonstrated a strong dependence on (002)/(100) ratios.