Designing phase‐adaptive UHTC multilayer composites: Layer width effects on mechanical and kinetic properties

The B1 rocksalt transition metal carbides and nitrides are among the materials with the highest melting temperatures. Additionally, members of this group of materials possess high‐temperature strength with ductility facilitated by diffusional creep. As a result, these are attractive materials for high‐temperature applications such as structural materials or coatings for hypersonic or space vehicles or within nuclear reactors. Despite their notable high‐temperature properties, these materials suffer from low fracture toughness at ambient temperatures, making them susceptible to catastrophic brittle failure during component manufacturing, assembly, and heating during end‐use applications. In this work, we explore metal–ceramic matrix composites with the ability to phase transform under the thermal load as a potential solution to improve low‐temperature fracture toughness while retaining high‐temperature mechanical properties. These ductile metal layers provide toughening to the composite at low temperatures when the ceramic is most susceptible to brittle failure by giving the potential to more than double the fracture toughness as compared to the equivalent monolithic ceramic. At elevated temperatures, an irreversible phase transformation occurs, resulting in the disappearance of metal layers, facilitated by nonmetal atom diffusion from ceramic layers to metal layers. As a result of the transformation, the metal layers, with their lower melting point, can now be completely removed to produce a ceramic phase that preserves the high‐temperature mechanical properties required for the intended end‐use application.