Silver Diffusion and High-Temperature Lubrication Mechanisms of YSZ-Ag- Mo Based Nanocomposite Coatings (Preprint)

摘要

Yttria-stabilized zirconia (YSZ) nanocomposite coatings consisting of silver and molybdenum were produced by a hybrid process of filtered vacuum arc, magnetron sputtering and pulsed laser depositions for tribological investigations at different high temperatures. The microstructure of the coatings was determined by X-ray diffraction and transmission electron microscopy. The friction coefficients were measured using a high-temperature ball-on-disk tribometer at 25 to 700(number sign)61616;C. The coatings with 24 at.% Ag and 10 at.% Mo contents showed a friction coefficient of 0.4 or less for all temperatures from 25 to 700&(number sign)61616;C. The wear scar surfaces and coating cross sections were studied using scanning electron, transmission electron, scanning transmission electron and focused ion beam microscopes, which provided the information on chemical composition distributions of silver and molybdenum along with microstructure features. The silver diffusion and coalescence on surfaces played an important part in the high-temperature lubrication mechanism of the YSZ-Ag-Mo coatings. Silver was found to be an effective lubricant at temperatures below 500&(number sign)61616;C and its coalescence on the surface isolated molybdenum inside coatings from ambient oxygen. Lubricious oxides of molybdenum were formed and lubricated at temperatures above 500&(number sign)61616;C when the silver was worn off the contact surface. For silver containment inside the coating at high temperatures, a multilayer architecture was build by inserting a TiN diffusion barrier layer in the composite coatings. Microscopic observations showed that this barrier layer prevented silver exit to the coating surface. At the same time, this enabled a subsequent lateral lubricant supply toward a wear scar location where the diffusion barrier layer was worn through and/or for a next thermal cycle. The multilayer coating maint.