Metastable Grain Boundary Configurations for Ultrahigh Strength Nanoceramics
Abstract
Ceramic materials are of key interest for ballistic protection of armored tactical platforms in the U.S. Army. However, to overcome the intrinsic brittleness found in monolithic parts, better fundamental understanding on the mechanisms of load accommodation must be established. Together with novel synthesis and processing strategies that can enable unique micro and nano-structural configurations exploiting those mechanisms, a new generation of materials with unmatched performances can be created. The present project aims to provide definitive fundamental understanding on the hardness and toughness of ceramics with an extensive grain boundary network, i.e. nanocrystalline ceramics, and to propose non-system specific toughening mechanisms that exploit the dependence of intergranular crack propagation on metastable grain boundary configurations. The mechanical behavior of nanocrystalline ceramics is undoubtedly dependent on the grain boundary characters. However, the dearth of understanding of the relationship between the local characteristics of grain boundaries and the observed macro-properties is still limited as demonstrated by the large number of contradictory trends reported on grain size dependences. For instance, while some authors have suggested the existence of a Hall-Petch dependence (increase of properties linearly with d-1/2, where d is grain size) down to grain sizes below 10nm, some report on the existence of an inverse relation for grain sizes below 100nm. Most of those contradictions emerge from the common presence of residual porosities and impurities in poorly processed ceramics, begging for systematic studies on controlled, pure and fully dense nanoceramics. Therefore, the first goal of this project is to develop a processing strategy in which impurity-free nanoparticles with grain sizes below 5nm are synthesized and sintered under high pressures (up to 2GPa) and high heating rates in a process called High Pressure Spark Plasma Sintering, targeting unprecedented retention of the grain sizes in the nanometer scale (<10nm) while provoking full densification. Materials representative of three crystalline structures will be consolidated, namely ZrO2 (cubic); ZnAl2O4 (cubic/spinel); and ZnO (hexagonal), and the mechanical properties tested as a function of grain sizes. The selection of these materials connects to the second goal of the project, which is to study the dependence of Hall-Petch slopes and inverse behaviors (if any) on the crystal structure and consequent grain boundary character population. Both hardness and toughness will be studied and the mechanisms analyzed by lifting areas of indented (for hardness) and cracked (for toughness) areas by using focus ion beam (FIB) and studying plastic deformation patterns at the atomic level and cracking dependences on grain boundary orientations and energies by utilizing high resolution electron microscopy and microcalorimetry. The third goal of the project is to develop a toughening mechanism that allows simultaneous improvement of hardness and toughness in nanoceramics independent of its crystal structure. To this end, we will exploit a recently discovered (result of a STIR project) connection between the total excess energies from grain boundaries and their spatial distribution and the fracture behavior to design crack propagation control. The effect of selected dopants segregated on grain boundaries on the formation of the space charge layer can enable metastable grain boundary configurations with unprecedented high cohesive strength to arrest crack propagation. More importantly, such metastable configurations are observed to energetically equalize the existing grain boundary population, causing controlled crack branching for unmatched toughness.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1810361
Entities
People
- Ricardo Castro
Organizations
- Army Contracting Command
- United States Army
- University of California, Davis