Atomic arrangement and mechanical properties of chemical-vapor-deposited amorphous boron

Amorphous boron can be synthesized by chemical vapor deposition (CVD) onto a tungsten wire substrate, and this core-shell fiber has been widely used in high-performance composites due to its superior mechanical properties. Although the amorphous boron coating makes a significant contribution to the high fracture strength, its mechanical properties have not been studied rigorously due to its thin thickness and strong adhesion to the substrate. Furthermore, the medium-range atomic ordering of CVD amorphous boron has not been clearly understood, and the determination of the closest crystalline structure has been a challenge. In this study, high-resolution transmission electron microscopy (HRTEM), nanoindentation, and in-situ micropillar compression were performed to investigate the atomic arrangement and mechanical properties. Electron diffraction and autocorrelation function analysis revealed α-rhombohedral boron ordering as the closest crystalline structure. Micropillar compression displayed near-ideal yield strength (~13 GPa), but nanoindentation a relatively moderate Young's modulus (~320 GPa), leading to a modulus of resilience (2.64 × 108 J/m3) unprecedentedly higher than most advanced engineering materials. Our structural and mechanical data will be discussed in terms of lattice point spacing and the influence of surface defects on fracture strength, respectively. Our results will be potentially useful to improve mechanical properties of amorphous boron core-shell fibers and related composites. [Display omitted] •Determination of atomic configuration and mechanical properties of amorphous boron synthesized by chemical vapor deposition•Medium-range order consisting of dominantly α-rhombohedral boron determined by HRTEM and auto-correlation image analysis•Hardness (~33 GPa) and Young’s Modulus (~317 GPa) determined by nanoindentation•In-situ scanning electron microscopy micropillar compression preformed to determine ultra-high yield strength (~13 GPa)•Ultra-high yield strength and moderate Young’s modulus lead to a modulus of resilience higher than most engineering materials