Preparation of chameleon coatings for space and ambient environments

Tribological coatings of yttria-stabilized zirconia (YSZ), Au, diamond like carbon (DLC) and MoS2 were synthesized using magnetron assisted pulsed laser deposition. The coatings were synthesized in four-component and three-component combinations that included YSZ/Au/DLC/MoS2, YSZ/Au/MoS2, and YSZ/Au/DLC. A range of coating compositions was studied to explore coating optimization for low friction in varying environments (dry, humid and high temperature). For four-component YSZ/Au/DLC/MoS2 coatings, the optimal compositions for friction adaptation between dry nitrogen and humid air included relatively high concentrations of the soft phase, Au (> 20 at.%), and low amounts of the hard phases, DLC and YSZ. Ex situ Raman spectroscopy analysis indicates that friction adaptation involves a combination of both lubricating species, MoS2 and carbon, where transitions of DLC to graphitic-carbon and amorphous MoS2 to its hexagonal phase occur after cycling between both room temperature humid air and dry nitrogen. In large carbon concentrations (> 30 at.%), the DLC component was found to be detrimental for friction in dry nitrogen and humid air, but promoted a longer coating wear life at 500 °C. The three-component coating of YSZ/Au/MoS2 performed well in both dry nitrogen and humid air, suggesting a synergism between Au and MoS2, where carbon was not necessary for lubrication in humid air.