Nanocomposite tribological coatings with “chameleon” surface adaptation

Abstract

Nanocomposite tribological coatings were designed to respond to changing environmental conditions by self-adjustment of their surface properties to maintain good tribological performance in any environment. These smart coatings have been dubbed “chameleon” because, analogous to a chameleon changing its skin color to avoid predators, the coating changes its “skin” chemistry and structure to avoid wear. The concept was originally developed using WC, diamondlike carbon, and WS2 material combination for adaptation to a humid/dry environment cycling. In order to address temperature variation, nanocomposite coatings made of yttria-stabilized zirconia (YSZ) in a gold matrix were developed with encapsulated nanosized reservoirs of MoS2 and diamondlike carbon (DLC). Coatings were produced using a combination of laser ablation and magnetron sputtering. They were characterized by x-ray photoelectron spectroscopy, x-ray diffraction, transmission electron microscopy, x-ray energy dispersive spectroscopy, and micro-Raman spectroscopy. Results were correlated with mechanical and tribological characterization. Coating hardness was evaluated using nanoindentation, while coating adhesion and toughness were estimated using scratch and Vickers indentation tests. Friction and wear endurance measurements of YSZ/Au/MoS2/DLC coatings against steel and Si3N4 balls were performed at room temperature in controlled humidity air, dry nitrogen, and vacuum environments, as well as at 500 °C in air. Depending on the environment, coating friction surface changed its chemistry and structure between (i) graphitic carbon for sliding in humid air [coating friction coefficients (c.o.f. 0.10–0.15)], (ii) hexagonal MoS2 for sliding in dry N2 and vacuum (c.o.f. 0.02–0.05), and (iii) metallic Au for sliding in air at 500 °C (c.o.f. 0.10–0.20). The unique coating skin adaptation realized with YSZ/Au/MoS2/DLC and WC/DLC/WS composites proves a universal applicability of the chameleon design concept.