Self-Lubricating Nanoporous Hard Coatings

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This research project proposes to explore and demonstrate a new approach to adaptive high-temperature lubrication. Self-organized nanopores within wear-resistant coatings are exploited to guide lubricant flow to the sliding contact surface. As a model system, this project uses sputter deposited CrN-Ag nanocomposite coatings where nanoporous CrN acts as a hard wear-resistant matrix which contains Ag, a low- and high-temperature solid lubricant. The project focuses on (a) understanding the nanopore formation mechanism, based on atomic shadowing effects and surface diffusion processes, by studying nanopore dimensions and Ag distribution as a function of deposition angle, growth temperature, ion-irradiation flux, (b) developing a kinetic model for lubricant-diffusion through 1-nm-wide nanopores, including geometrical constraints for transport and agglomerate formation within the coating, using annealing experiments and compositional mapping, and (c) studying the adaptive lubrication mechanisms at elevated temperatures, by mapping friction and wear over a space of temperature, contact pressure, and sliding speed.This research program will provide a fundamental scientific understanding, including coating deposition, lubricant transport, and tribological mechanisms, for the investigated new approach for self-lubrication. The new approach uses self-assembled 1-nm-wide pores, that form in transition-metal nitride hard-coatings, as channels that guide the diffusive transport of solid lubricants. During high-temperature operation, a solid lubricant which is contained in the hard matrix will move through the pores to replenish a lubricious surface-layer. This new strategy is applicable to a range of coating systems and has the potential to become a breakthrough technology by providing low friction surfaces in various environments and during multiple temperature cycles ranging from 0 to ~1000 C. Applications include hard wear-resistant lubricious coatings for high-temperature bearings in fuel-efficient jet-engines and gas-turbines, solid-lubrication in cyclic air-vacuum environments for space applications, and oil-free air-foil bearings in gas compressors for fuel cells.