James A. Wollmershauser

Below the Hall–Petch Limit in Nanocrystalline Ceramics

Reducing the grain size of metals and ceramics can significantly increase strength and hardness, a phenomenon described by the Hall-Petch relationship. The many studies on the Hall-Petch relationship in metals reveal that when the grain size is reduced to tens of nanometers, this relationship breaks down. However, experimental data for nanocrystalline ceramics are scarce, and the existence of a breakdown is controversial. Here we show the Hall-Petch breakdown in nanocrystalline ceramics by performing indentation studies on fully dense nanocrystalline ceramics fabricated with grain sizes ranging from 3.6 to 37.5 nm. A maximum hardness occurs at a grain size of 18.4 nm, and a negative (or inverse) Hall-Petch relationship reduces the hardness as the grain size is decreased to around 5 nm. At the smallest grain sizes, the hardness plateaus and becomes insensitive to grain size change. Strain rate studies show that the primary mechanism behind the breakdown, negative, and plateau behavior is not diffusion-based. We find that a decrease in density and an increase in dissipative energy below the breakdown correlate with increasing grain boundary volume fraction as the grain size is reduced. The behavior below the breakdown is consistent with structural changes, such as increasing triple-junction volume fraction. Grain- and indent-size-dependent fracture behavior further supports local structural changes that corroborate current theories of nanocrack formation at triple junctions. The synergistic grain size dependencies of hardness, elasticity, energy dissipation, and nanostructure of nanocrystalline ceramics point to an opportunity to use the grain size to tune the strength and dissipative properties.

Growth per cycle of alumina atomic layer deposition on nano- and micro-powders

Core–shell powders consisting of a tungsten particle core and thin alumina shell have been synthesized using atomic layer deposition in a rotary reactor. Standard atomic layer deposition of trimethylaluminum/water at 150 °C utilizing a microdosing technique was performed on four different batches of powder with different average particle sizes. The particle size of the powders studied ranges from ∼25 to 1500 nm. The high mass-thickness contrast between alumina and tungsten in transmission electron microscopy images demonstrates that the particle core/shell interface is abrupt. This allows for the uncomplicated measurement of alumina thickness and therefore the accurate determination of growth per cycle. In agreement with prior works, the highest growth per cycle of ∼2 A/cycle occurred on the batch of powder with the smallest average particle size and the growth per cycle decreased with increasing average particle size of a powder batch. However, the growth per cycle of the alumina film on an individual pa...

Growth mode of alumina atomic layer deposition on nanopowders

Alumina films were grown by atomic layer deposition in a rotary reactor on tungsten nanopowder with an average particle size of 54 nm. Films of various thicknesses were formed using trimethylaluminum and water at a reaction temperature of 110 °C by varying the number of deposition cycles from 2 to 78. The sharp contrast between the deposited alumina shell and the tungsten core in transmission electron microscopy allows for easy film thickness measurements and determination of the film thickness as a function of the deposition cycle. The growth curve shows that the rate of film thickness increase does not follow a single linear response but instead consists of three characteristically different growth phases. These phases occur in different deposition cycle regimes: phase I occurs in ≤5 cycles, phase II between 5 and ∼15 cycles, and phase III begins in ∼15 cycles and continues until at least 78 cycles. The average growth per cycle for phases I, II, and III is ∼4.5, <1, and 2 A/cycle, respectively. The aver...

Truly Nanostructured Transparent Ceramics - New Properties and Opportunities

NRL technology has produced transparent MgAl2O4 ceramics with hardness ~20 GPa and grain sizes less than 30nm; 50 % improvement in hardness and an order of magnitude reduction in grain size than present literature best.