Grain-boundary and interface segregation as an effective tool to enhance strength and toughness of nanostructured ceramics
Despite the knowledge we have on the structure-property relations in solids, it is still a challenging task to synthesise materials which are exceptionally strong and tough at the same time. This strength-toughness trade-off is a long-standing dilemma in materials science, which limits the potential of many structural materials. In this work, we show a perspective way of enhancing strength of nanostructured AlCrN ceramic thin films by controlled segregation of AlN-rich precipitates at the grain boundaries and interfaces intentionally formed during the processing by compositional variations across the film thickness. The segregation of AlN-rich precipitates was controlled by annealing at a temperature of 1100°C, which was sufficiently high to induce phase decomposition of the metastable AlCrN solid solution. The size, distribution and composition of the precipitates formed at the grain boundaries and interfaces were studied by electron microscopy and atom probe tomography. The micromechanical testing of cantilever beam specimens fabricated by focus ion beam revealed an enhancement of both strength and toughness of the annealed film. A detailed study of the fracture surfaces indicates that the evasion of strength-toughness trade-off is due to inter-granular fracture, typical for brittle polycrystalline ceramics, turning to trans-granular fracture as a result of (i) an interaction of the crack tip with the elastic and ductile precipitates and (ii) a change of the energy landscape and cohesive strength of the grain boundaries and interfaces decorated by the AlN-rich phase.