Grain Boundary Complexions and Complexion Transitions in Boron Suboxide Processed with Yittria-Silica Based Additives (Core Competency 1.1.2 - Advanced Materials Processing)
Abstract
Most ceramic materials are polycrystalline and the interfaces between grains, or commonly referred to as grain boundaries, have a dominant effect on material properties. Due in part to the development of advanced sub-Angstrom electron and atom-probe microscopy capabilities in the past twenty years, recent work has shown that the composition and structure of the grain boundaries can be engineered to improve material properties by using thermodynamic diagrams (i.e. equilibrium diagrams) and kinetic diagrams (time-temperature-transformation diagrams) that are specific to grain boundaries. The study and design of grain boundaries is now widely accepted as the field of grain boundary complexions wherein a complexion is defined as the equilibrium state of an interface. The majority of grain boundary complexion research has focused on alumina as a model system, and using the concepts developed during the study of alumina, this grant agreement intends to support the development of advanced armor ceramics for personnel protection by leveraging grain boundary complexions and their transitions to design an improved generation of boron suboxide (B6O) ceramics. The main objective of this work is to process boron suboxide with various yttria-silica or rare-earth oxide-silica based sintering additives and evaluate the role of the sintering additives on grain boundary structure and composition with the end goal to be to optimize microstructural development and mechanical properties. A secondary objective will be to develop grain boundary complexion diagrams and complexion time-temperature-transformation diagrams of these material systems to assist future development of these materials. Sintered boron suboxide ceramics will be supplied by ARL. The bulk ceramics will be sintered by hot-pressing techniques and the microstructures will be characterized initially using scanning electron microscopy techniques. Advanced aberration-corrected scanning transmission electron microscopy (STEM) will then be utilized to determine grain boundary structures and compositions as a result of different sintering additives and heat treatments. Abnormal grain growth in particular will be tracked as a proxy for a grain boundary complexion transition. ?-factor microanalysis, a quantitative energy dispersive spectroscopy framework that enables X-ray absorption corrections, will be employed to determine the excess planar coverages of various grain boundary segregants. The mechanical response of different grain boundary complexions will first be evaluated using VickerÕs microhardness indentation to induce cracks where the crack path morphologies will be evaluated to qualitatively investigate crack deflection as a fracture resistance mechanism. Finally, small-scale microcantilever beam testing may be employed to measure the grain boundary toughness of different complexion types. Overall, the field of materials science and engineering lacks a direct and quantitative understanding of grain boundary structure-property relationships, especially for grain boundaries in materials that are serviced in extreme environments (i.e. armor ceramics). Furthermore, while extremely useful, there are limited examples of grain boundary complexion diagrams and complexion time-temperature-transformation diagrams outside of alumina. This research will develop and apply meticulous experimental tests to directly quantify grain boundary structure-property relationships and also develop and showcase new varieties of complexion diagrams. Upon successful completion of this research, these new methods can be applied to other material systems and similar complexion diagrams can be formulated.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Apr 22, 2019
- Source ID
- W911NF1710079
Entities
People
- Martin P Harmer
Organizations
- Army Contracting Command
- Lehigh University
- United States Army