Multiscale modeling research tackles superhard problems

A collaboration between two assistant professors with expertise in computational modeling aims to improve the ductility of superhard materials like boron carbide, paving the way for their implementation in applications such as body armor or cutting tools.

Qi An

Qi An

Lei Cao

Lei Cao

Qi An, assistant professor of materials science and engineering, and Lei Cao, assistant professor of mechanical engineering, were recently awarded a National Science Foundation grant for their project titled "Modeling and Design of Enhanced Strength and Ductility though Grain Boundary Engineering - A study of Boron Carbide Based Superhard Materials."

Boron carbide, the third hardest material in the world, has a number of potential engineering applications due to its combination of low density, high thermal stability and high abrasion resistance. However, its low ductility limits its use in key applications such as body armor for soldiers or cutting tools.

The research by An and Cao uses multiscale computer modeling to better understand the failure mechanisms of boron carbide and develop design principles that could improve its flexibility.

Their research focuses on the grain boundary - a place where two materials with different textures meet. Grain boundaries, which are plentiful in ceramics such as boron carbide, control the mechanical properties of a material, so understanding how they react under different loads could be a key to developing ways to improve the ductility of boron carbide.

Multiscale approach provides models, possible solutions

Although their research is in the early stages, An and Cao have some ideas about how boron carbide cracks.

"Based on our current studies, the mechanism of failure is the grain boundary sliding," An said. "You can imagine when you apply stress, the grain will slide, so that will create an amorphous region with higher density. If you imagine higher density in this small area, the crystal part is lower density, it creates a cavity. The cavity will grow and can crack."

An, whose expertise is in atomic-level modeling, will lead the first portion of the project, focused on understanding the atomic structure and physics of the grain boundaries. An will be using a modeling method called reactive force field modeling capable of simulating the bond breaking involved in the fracture process of superhard materials. An's goal will be to describe the underlying failure mechanisms at the grain boundaries, a necessary first step before the team can begin to propose engineering solutions to improve ductility.

"We cannot propose anything because there's no modeling capability that can predict the materials accurately right now. Without understanding how it fails, you cannot come up with a recipe or principles, you cannot make the materials better," Cao said. "So first we will construct this predicting capability, and once we understand this, we can try different alloying strategies to see if it will make it more ductile."

Once the underlying physics are resolved, Cao will conduct phase field modeling that more realistically simulates experimental-scale conditions.

"Qi can describe the grain boundaries very realistically with all the physics included. However, the domain size is pretty small and the structure is relatively simple compared to experimental conditions," Cao said. "So he resolves all the detailed physics, and I can do a polycrystal simulation that is similar to the experimental condition, and then I predict how the material will react to the deformation and if our grain boundary engineering is successful or not."

Research paves way for future collaborations

The results of this research may be applicable to other superhard materials, such as boride and carbide, diamond, and cubic boron nitride - all materials in which grain boundaries play a critical role in determining the properties of the materials.

An and Cao say understanding how to control the strength and ductility of superhard materials like boron carbide would be a major breakthrough for both researchers and manufacturers looking to use superhard materials.

An and Cao, who both joined the University of Nevada, Reno in 2016, hope to continue their collaboration in future projects that emphasize multiscale modeling, but applying their techniques to different materials, such as metals used in manufacturing applications.