Mechanical Properties and a Brittle-to-Ductile Fracture Transition in 3D Boron Nitride Foams.

We uncover the fracture characteristics of a boron nitride foam (BNF): a highly promising nanomaterial with a large band gap, superelastic behavior, and high surface area. By applying tension tests to BNF samples and characterizing them using image-processing tools and detailed scanning and transmission electron microscopies, we demonstrate a transition from brittle to a ductile fracture. Complementary mechanical analyses revealed that constraints originating from the synthesis process induce significant prestresses in the BNF and that wall thickness variations explain the fracture transition. We also show that BNF has a nearly zero Poisson's ratio and a high (>200 MPa) shear strength and that it absorbs a significant amount of energy before the fracture occurs. Thus, our findings shed light on the fundamental microscopic-scale mechanics of BNF, paving the way toward its integration into advanced applications, such as wearable electronics and energy absorbers.