Grain Boundary Polarization to Increase Toughness of Nanocrystalline Transparent Spinel
Technical Report,01 Jan 2017,30 Sep 2017
University of California - Davis Davis United States
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Transparent ceramics are of key interest for ballistic protection of armored tactical platforms in U.S. Army applications. MgAl2O4 spinel is among the materials of choice because of its high light transmittance, and chemical, mechanical, and thermal stabilities. The mechanical properties of polycrystalline spinel are highly dependent on its microstructure and grain size refinement has been proven to enhance hardness to unprecedented levels. While we have recently demonstrated that grain sizes of 14 nm can lead to hardness above 27GPa Vickers, these superstrong nanoceramics at ambient temperature show low fracture toughness, limiting their practical utility. Although progress has been made in enhancement of toughness with composites, for applications where transparency is required this has to be done without the inclusion of second phases such as in dispersion strengthening that causes light scattering. This means that the properties need to be tailored by controlling atomistic mechanisms underlying the failure of the single phased material. Mechanical behavior of nano-ceramics is strongly dependent on the grain boundary characters. However, the dearth of understanding of this relationship between the local characteristics of grain boundaries and the observed macro-properties hinders proper materials design. In this project we propose a combination of novel synthesis and processing strategies to allow refined control of atomic structure and bonding at grain boundaries of single-phased nanostructured spinel and cubic zirconia. The goal of the strategies is twofold 1 strengthen grain boundaries by using dopants prone to segregation. Very small amounts of rare-earth elements are expected to segregate to grain boundaries due to elastic and electrostatic forces and form more bonds between neighboring grains than the host ions lowering the grain boundary energy.
- Properties of Metals and Alloys
- Electricity and Magnetism