Design of smart transforming metal ceramic multilayers

Abstract

Novel metal ceramic multilayer composites with the ability to phase transform into a single phase ultra-high temperature ceramic are described. These materials offer enhanced low-temperature fracture toughness and reliability without sacrificing high-temperature mechanical properties by eliminating the metal reinforcement layers upon reaching temperatures sufficient for the activation of carbon diffusion. At low temperatures, these materials are practically indefinitely stable because of the high activation energy for carbon diffusion in the carbide layers that control the phase transformation kinetics. Here, knowledge of the overall thermodynamics, i.e., carbon content, is combined with prediction of phase transformation kinetics and toughening in the limit of small-scale yielding to characterize the design space of these novel composites. It is proposed that the ability to control thermodynamic and mechanical properties of the post-transformation ceramic via selection of metal-to-ceramic layer ratios makes these novel composites an excellent alternative to current methods for reinforcement of ultra-high temperature ceramics.