Thermal stability of grain size and hardness in nanocrystalline spinel
Savannah Ullrich is pursuing her PhD in the Materials Science program. She is working with the Naval Research Laboratory to determine the thermal stability and hardness of transparent nanocrystalline magnesium aluminate spinel. In December 2016, she graduated with a Bachelor of Science in Chemistry from the University of Arkansas where she was a Fry Scholar. Savannah then performed product testing within the Chemistry Department at Consumer Testing Laboratories prior to joining the Mines community in August, 2017. She has past experience in metal and hybrid nanoparticle synthesis.
Magnesium aluminate spinel has been developed for transparent-ceramic windows and armor applications and is known to exhibit the Hall-Petch relationship, whereby reducing the grain size increases hardness. Recent investigations of grain-size effects in spinel, with extremely small grain sizes ranging from 3.6 nm to 37.5 nm, find that spinel maintains a Hall-Petch hardness relationship down to 18.4 nm grain size. Fully dense, additive-free, nanocrystalline spinel samples are produced by the Naval Research Laboratory using an environmentally controlled pressure-assisted sintering technique and pressless multianvil split sphere apparatus to suppress grain growth and promote sintering. Nanocrystalline pure metals, such as copper and aluminum, have been shown to undergo stress-induced grain growth at temperatures near 20 percent of their melting temperatures; it is unknown if similar stress-induced events occur in nanocrystalline spinel. In this work, the thermal stability of nanocrystalline spinel with respect to grain growth is evaluated using x-ray diffraction at moderate temperatures up to 400 °C. No changes in grain size were detected at this temperature, which is 28 percent of the spinel melting temperature. Hardness and modulus are characterized using high-temperature nanoindentation over the same grain size and temperature range. This work is part of a larger study to determine the thermal stability of nanostructured spinel and its properties up to and beyond 400 °C. Another objective of this work is elucidating the deformation principles and behavior behind the Hall-Petch relationship and breakdown in nanocrystalline ceramics through this study of mechanical properties as a function of both temperature and grain size of spinel.