Month: August 2020

Jessica passed her dissertation proposal! Congratulations!

Jessica passed her dissertation proposal! Many Congratulations!!!


Title: Influence of grain boundaries on the mechanical properties of ceramic materials with high interface density


The development of lightweight, high strength, cost-effective transparent materials is in high demand for military vehicles, vessels, electronics, and sensor applications. Recent advances in the synthesis of transparent nanocrystalline ceramics have made it a superior choice over conventionally used glass due to their excellent mechanical properties, but their plasticity and fracture mechanisms have not been clearly understood, yet. In our preliminary study, transparent magnesium aluminate spinel with grain sizes ranging from 3.7 to 80 nm has been successfully synthesized using environmentally controlled pressure-assisted sintering. Hardness measured by nanoindentation has revealed a breakdown of the Hall-Petch relationship at a critical grain size of 18.5 nm with a measured hardness of 22.5 GPa. Below the critical grain size, as the grain size decreases, hardness decreases. This result indicates the emergence of a new plasticity mechanism such as grain boundary sliding. Micropillar compression was implemented to determine the yield strength as a function of grain size. Unlike the hardness dependence on grain size, below the critical grain size, yield strength did not decrease but rather remains the same. In addition, fracture occurred with no plasticity, indicating that a grain boundary would serve as a crack source, which is different from its role as a source of plasticity under nanoindentation. Our preliminary results clearly show that grain boundaries affect the mechanical behavior differently under different deformation condition. In order to gain a deeper understanding of the mechanical behavior of nanocrystalline ceramics, therefore, it is critical to understand the role of grain boundaries in plasticity and fracture processes under a given deformation mode.

In this proposal, therefore, we propose to perform experimental and computational studies to understand the effects of grain boundaries on the mechanical properties of nanocrystalline ceramics under various deformation modes. Micromechanical testing will be used to characterize their mechanical response under indentation, compression, bending, and fatigue. Electron microscopy will be performed to examine the evolution of defect structures before and after mechanical tests. Constitutive modeling and atomistic simulation will also be performed to understand the grain-boundary-assisted mechanisms that govern plasticity and fracture processes. The successful completion of this investigation will provide a fundamental understanding of the influence of grain boundaries on the mechanical properties of transparent ceramics and allow for the more rapid development of transparent ceramics with improved mechanical properties.