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Boron carbide is a ceramic often used in body armor because of its hardness and density, as well as its properties related to heat and material changes in composition and structure. So researchers at the University of Florida (UF) recently used Expanse at the San Diego Supercomputer Center (SDSC) at UC San Diego to investigate stress effects on this useful but naturally brittle material at the atomic level. Running molecular dynamics simulations powered by Expanse, the scientists illustrated bond breakage from imposed tensile shock, or stretching. The results have been published in the European Journal of Mechanics - A/Solids.

“Shock response of boron carbide under compressive shock has been well-explored using molecular dynamic simulations, but most of these studies explore phase transformations, pressure-volume relationships (also called Hugoniot) and thermal effects (temperature rise) due to the compressive nature of shock loading,” UF Professor Ghatu Subhash explained. “However, the fracture behavior of ceramics is dominated by tensile stresses and no atomistic level investigations were currently available, hence this lack of knowledge motivated us to explore a novel method to develop a new procedure to impose tensile shock on a domain of boron carbide crystal in molecular dynamic simulations and investigate the bond breakage behavior.” 

Subhash and his team, Amith Adoor Cheenady and Amnaya Awasthi, used their Expanse-generated molecular dynamics simulations to better understand the strong covalent bonds that form boron carbide’s network of 41 bonds among 15 atoms. They developed a new method to quantify strains in all these bonds as the crystal is deformed. Their work revealed that at the atomistic scale the stress-strain response of boron carbide crystals exhibits traits – marked by extended plastic strain – that are characteristic of ductile materials, or those that can be changed in form without breaking. The researchers found that when boron carbide was loaded along another crystal orientation perpendicular to the first, the stress-strain response was characteristic of brittle materials, marked by a rapid drop in stress beyond the ultimate strength with minimal plastic deformation.

Onset of fracture in boron carbide due to intersection of two tensile shock waves. The atoms are colored by atomic strain, with the red atoms being the highest strained and blue being the lowest. Credit: Amith Adoor Cheenady, Department of Mechanical and Aerospace Engineering, University of Florida

“We were surprised with the rapid drop in stress beyond the ultimate strength with minimal plastic deformation,” Subhash said. “These stark differences in the mechanical response of boron carbide were one of the most exciting findings from our study.”

These investigations utilizing SDSC’s Expanse demonstrated an important difference in exploring the fracture behavior due to tensile stress in boron carbide as opposed to the phase transformations and thermal effects due to the compressive nature of shock loading. The breakthrough allowing for the quantification of the breaks in the bonds allowed for a weak link to be found within the boron carbide bonds. Understanding the breakage mechanisms, mechanical response and weak links will allow for the strengthening of this material in future design efforts.

In the future, Subhash said that he and his team will look into the development of machine learned interatomic potentials (MLIPs) for boron carbide, silicon carbide and boron suboxide – a similar material to boron carbide – with the benefit of being harder at a low density. They will investigate the atomic structures to assess the MLIPs in similar applications as boron carbide.

This work was supported by the Army Research Office (grant no. W911NF1810040). The computations were conducted on Expanse with National Science Foundation Extreme Science and Engineering Discovery Environment (allocation no. MSS160016).