Structure and Micro-Fracture Testing of Single Grain Boundaries in Ceramics

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The research objective of this grant is to elucidate the sub-nanometer structural and chemical characteristics of transparent magnesium aluminate spinel grain boundaries that are responsible for intergranular fracture when LiF is used as a sintering aid. It has long been known that partially-disordered intergranular films (IGFs) at the 1-2 nm scale can have a profound effect on the behavior of ceramic materials, and it is now becoming evident that newly-revealed thinner layers (complexions) can have similar effects on macroscopic behavior. Although conventional High-Resolution TEM studies have not identified residual LiF in fully-processed liquid phase sintered spinel, light elements like Li and F are not detectable by this technique in the quantities typical of many boundary complexions so there is high likelihood that sub-nanometer layers of boundary LiF are, indeed, behind the poor boundary strength. The proposed program will test this proposition in a highly coordinated set of experiments correlating the mechanical properties of doped grain boundaries with their associated grain boundary chemistry and structure. This will be accomplished through the marriage of cutting-edge aberration-corrected Scanning Transmission Electron Microscopy (ac STEM) and newly-developed microscale fracture testing techniques that are suitable for evaluating the local fracture properties of boundaries.The results of this program will inform the development of transparent, fracture-resistant spinel materials for spacecraft windows, sensor domes, armor, and protective goggles and face shields. Although the study of interface complexions is in its early days, the long-term impact of the proposed work is through the contribution of a key element (the link between boundary fracture behavior and complexion) to a developing scientific framework for documenting, understanding and exploiting complexions in ceramic materials. There is great potential to achieve unique combinations of properties in many ceramic (and even metallic) systems by tailoring grain boundary structure and chemistry according to the complexion framework. The development of micro-scale fracture techniques that can be performed using commercially-available instruments will present, for the first time, the opportunity for a large number of people and industries to be directly involved in carrying out quantitative fracture measurements with very small volumes of material. Impact on human development will be primarily through the education and training of the graduate students and undergraduates who will be involved with the project, who will have access to the latest microscopy and microtesting technologies. Impact on the local community will be achieved through continuing participation in outreach activities including ?NanoDays? at the Da Vinci Discovery Center of Science and Technology in Allentown, PA.