Correlating Grain Boundary Character to Micro-Fracture in Transparent Ceramic Materials

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Transparent spinel is an advanced ceramic material that has great potential for use as a highly damage-resistant window material. It is fabricated by pressing individual grains of ceramic powder together at elevated temperature until they fuse to form a solid with excellent optical transparency and extreme hardness. Unfortunately, forming windows of practical size at an affordable cost is a significant challenge. This award supports fundamental research focused on a new approach to understanding and controlling the characteristics of a ceramic material. In particular, the work will explore the way that atoms arrange themselves at the boundaries between the individual grains during and after processing, and how different arrangements affect the material's strength. Improvements in spinel processing will benefit aerospace and military applications by providing a cost-effective solution when damage-resistant windows are needed. Such windows will offer improved protection for soldiers, sailors, pilots, and astronauts. Two female graduate students will be trained to work with cutting-edge techniques and tools. Undergraduate students will be engaged through academic-year and summer research projects. Benefits to the local community will take place through outreach events aimed at K-8 students.Techniques such as aberration-corrected electron microscopy are now revealing that grain boundaries and interfaces constitute distinct thermodynamic states of matter that have consistent three-dimensional structures at the atomic level. These structures, termed complexions, are defined by thermodynamic variables such as chemical potential, temperature, pressure, and crystallography of the abutting grains. Furthermore, there are strong indications that control of boundary complexions is a method by which bulk properties may be controlled. In order to accelerate the use of complexion engineering as a tool for improving material properties, this research project is focused on the creation and validation of interface complexion time-temperature-transformation diagrams. These diagrams will be used to relate complexion types to interface fracture behavior. Aberration-corrected electron microscopy and small-scale mechanical test techniques will be used to shed light on the character and behavior of individual pure and doped boundaries. The demonstration that it is feasible to create and utilize boundary transformation diagrams, and that controlling complexions can also control boundary fracture behavior, will provide the ceramics community with a new means of engineering bulk material behavior.