[Image above] UC Davis researchers found that adding extra aluminum to zinc aluminate can extend the Hall-Petch relation. Credit: Pxfuel
While “go big or go home” may be the mantra of many sports teams, “the smaller, the better” is the goal for many materials scientists in recent years.
Ever since development of the scanning tunneling microscope in 1981, nanomaterials and nanotechnology have experienced a meteoric rise in popularity. When researchers design materials on this scale, they can often achieve vastly different material properties compared to the bulk sample.
For ceramics, the ability to understand what is happening on the nanometer scale is especially useful. Ceramics are made up of millions of small crystals, or grains, arranged in different orientations. Being able to precisely arrange these building blocks has led to some important discoveries, for example, extending the Hall-Petch relation.
The Hall-Petch relation is a quantitative description of how yield strength of a material increases as grain size decreases. In other words, a ceramic becomes harder as its grains become smaller.
When the Hall-Petch relation was first described in the 1950s, scientists did not yet have the techniques to explore how far this relationship could go. But in the decades following 1981, development of techniques for designing on the nanoscale opened the door to this exploration.
In 1989, an open-access paper by Chokshi et al. observed what would come to be known as the inverse Hall-Petch relation. The inverse Hall-Petch relation is the term for a phenomenon that occurs once grains become smaller than 100 nm. Instead of the ceramic becoming harder, researchers have witnessed what they call “softening,” i.e., its hardness begins to decrease.
Researchers for years have debated the mechanisms behind inverse Hall-Petch in ceramics. In 2019, a team led by Ricardo Castro, ACerS Fellow and professor of materials science and engineering at the University of California, Davis, suggested that cracking caused by “the activation of new energy dissipation modes” was the cause.
In a CTT post on that study, Castro said the next step was to define a strategy to control cracking and thereby avoid the inverse Hall-Petch relation. Now, in a paper published almost exactly two years later, Castro and graduate student Luis Sotelo Martin, a member of ACerS President’s Council of Student Advisors, describe one approach to mitigate inverse Hall-Petch and extend the Hall-Petch relation.
They begin by recounting what they determined in the 2019 study—that as grain sizes decrease, the corresponding increase in grain boundary energy leads to intergranular fracture and a decrease in hardness.
“This proposed relationship between grain boundary energy (or grain boundary strength) and Hall-Petch behavior suggests that the inverse Hall-Petch relation could be mitigated by avoiding an increase in grain boundary energy, suggesting ‘colossal’ hardening may be achieved in nanoceramics,” they write.
So, how can an increase in grain boundary energy be avoided? The researchers suggest off-stoichiometry could be a solution.
Off-stoichiometry refers to compounds that do not have the expected integer ratios between components. For example, an off-stoichiometric compound of zinc aluminate (ZnAl2O4) containing extra aluminum would be ZnAl2.87O4.
Previous studies found that ceramics featuring off-stoichiometric formulations have altered grain boundary properties. For example, studies on magnesium aluminate show excess aluminum affects the local space charge layer and phenomena such as grain boundary migration. Also, a study on zinc aluminate showed that increasing the ratio of aluminum to zinc can suppress coarsening during sintering.
Despite its potential for altering grain boundary properties, the effect of off-stoichiometry has not been well-studied in the context of the Hall-Petch relation. Castro and Sotelo Martin looked to fill that gap with their recent study, which was funded by the Army Research Office.
They chose zinc aluminate nanoceramics as the basis for their research because of its possible applications in industry.
“Zinc aluminate is a fantastic candidate for transparent armor and laser hosts,” Castro explains in an email. “It has much higher thermal conductivity than other candidates such as sapphire, magnesium aluminate, or ALON. Thermal conductivity is directly related to thermal shock resistance, so ZnAl2O4 is a winner. Also, it is not more difficult to process than magnesium aluminate.”
They used co-precipitation to synthesize a quasi-stoichiometric nanocrystalline ZnAl2.01O4 powder and an aluminum-rich ZnAl2.87O4 nanopowder. Then, they sintered the nanopowders in one of two ways—deformable-punch spark plasma sintering and high-pressure spark plasma sintering.
They sintered the nanopowders two ways to address a question arising from previous studies. Specifically, some studies that fabricated ceramics via high-pressure spark plasma sintering observed the occurrence of inverse Hall-Petch, while some studies that fabricated ceramics via deformable-punch spark plasma sintering did not. Because these results could indicate an effect of sintering condition on the inverse Hall-Petch relation, Castro and Sotelo Martin wanted to account for this possibility.
Following Vickers hardness testing, the researchers determined that sintering technique does not substantially affect the occurrence of inverse Hall-Petch in nanocrystalline oxides. In contrast, excess aluminum did have an effect—while the quasi-stoichiometric samples prepared by high-pressure spark plasma sintering exhibited a maximum hardness of 18.6 GPa at a grain size of 21.4 nm, the aluminum-rich samples produced by the same technique strengthened up to 19.2 GPa at 12.6 nm grain sizes.
In the discussion section, Castro and Sotelo Martin explain that analysis of the samples showed aluminum enrichment altered the cracking patterns formed beneath the Vickers indentations, which affects how the samples accommodate load. Specifically, the aluminum “improves grain boundary toughness by serving as a pinning agent for grain shearing, similar to reports for rare-earth doping,” they write.
Castro says they hope this study will inspire further research in the ceramics community.
“There is no reason to believe this is only related to nanoceramics, as properties of microceramics also depend on grain boundary properties,” he says. In addition, “We have some preliminary tests showing that toughness can also be improved in transparent nanoceramics with similar mechanisms, and we are expanding this concept now.
The paper, published in Journal of the American Ceramic Society, is “Al excess extends Hall-Petch relation in nanocrystalline zinc aluminate” (DOI: 10.1111/jace.18176).