UC Davis

Refining grains to a nanoscale, specifically to dimensions of 40 nm or less, is associated with a significant increase in the grain boundary area. Consequently, specific deformation mechanisms, less critical in coarser structures, become more pronounced. Nanocrystalline ceramics display mechanical properties that differ significantly from their macro-scale counterparts. This dissertation delves into these properties, emphasizing the influence of thermodynamics at the interfaces on the mechanical properties of nanocrystalline ceramics.Achieving full-density ceramics while maintaining nanoscale grains is challenging due to the competitive nature of grain growth. Effective strategies for producing nanocrystalline ceramics encompass the application of high external pressures. These pressures boost densification, decrease the temperature, and shorten sintering time. However, these methods impose constraints on the size and geometry of fully dense nanoceramic parts. Consequently, obtaining samples with required dimensions for mechanical testing and scaling up production becomes exceedingly difficult. In the research, sodium was introduced to achieve a 42% reduction in the pressure needed for fully dense nanocrystalline magnesium aluminate. Sodium is a potent sintering aid in magnesium aluminate spinel (MAS) and enhances its mechanical properties. Through a combination of molecular dynamic simulations and experimental research, it was discovered that sodium strengthens the grain boundary, resulting in increased hardness and indentation toughness in MAS. The study investigates the role of dopants in load-bearing mechanisms, such as quasi-plasticity and crack propagation. As the grain size of spinel decreased from 500 µm to 24.4 nm, an 86% increase in flexural strength was observed. Moreover, a direct correlation between fracture toughness and flexural strength in fully dense MAS was identified, and the influence of external factors like strain rate on flexural strength was assessed. The YSZ project advanced our comprehension of nanocrystalline transparent ceramics, revealing that stabilizing grain boundaries with gadolinium dopants significantly enhances ceramic hardness. This research confirms that the Hall-Petch breakdown can be postponed, and hardness can be tailored by stabilizing grain boundaries. It was observed that crack formation and propagation are the primary load accommodation mechanisms in grain sizes where the inverse Hall-Petch relationship is evident. Furthermore, a direct relationship was identified between grain boundary energy and hardness in yttria-stabilized zirconia (YSZ), with a marked increase in grain boundary energy corresponding to the Hall-Petch breakdown. Adjusting grain boundary energy using gadolinium dopants resulted in increased resistance to crack propagation and augmented hardness, emphasizing the potential of strategic dopant addition in modifying ceramic properties. In conclusion, this comprehensive study highlights the potential of nanoceramics in various applications, from armor production to transparent windows, paving the way for future advancements in the field.