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.

Electron backscatter diffraction study of hexagonal boron nitride growth on Cu single-crystal substrates.

Hexagonal boron nitride (h-BN) is an important material for the development of new 2D heterostructures. To enable this development, the relationship between crystal growth and the substrate orientation must be explored and understood. In this study, we simultaneously grew h-BN on different orientations of Cu substrates to establish the impact of substrate structure on the growth habit of thin h-BN layers. The substrates studied were a polycrystalline Cu foil, Cu(100), Cu(110), and Cu(111). Fourier transform grazing-incidence infrared reflection absorption spectroscopy (FT-IRRAS) was used to identify h-BN on copper substrates. X-ray photoelectron spectroscopy (XPS) was used to determine the effective thickness of the h-BN. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to measure the morphology of the films and postgrowth crystal structure of the Cu substrates, respectively. Combining the SEM and EBSD images allowed for the correlation between h-BN film coverage and the crystal structure of Cu. It was found that the growth rate was inversely proportional to the surface free energy of the Cu surface, with Cu(111) having the most h-BN surface coverage. The Cu foil predominately crystallized with a (100) surface orientation, and likewise had a film coverage very close to the Cu(100).

Growth and spectroscopic characterization of monolayer and few-layer hexagonal boron nitride on metal substrates.

Atomically thin two dimensional hexagonal boron nitride (2D h-BN) is one of the key materials in the development of new van der Waals heterostructures due to its outstanding properties including an atomically smooth surface, high thermal conductivity, high mechanical strength, chemical inertness and high electrical resistance. The development of 2D h-BN growth is still in the early stages and largely depends on rapid and accurate characterization of the grown monolayer or few layers h-BN films. This paper demonstrates a new approach to characterizing monolayer h-BN films directly on metal substrates by grazing-incidence infrared reflection absorption spectroscopy (IRRAS). Using h-BN films grown by atmospheric-pressure chemical vapor deposition on Cu and Ni substrates, two new sub-bands are found for the A2u out-of-plane stretching mode. It is shown, using both experimental and computational methods, that the lower-energy sub-band is related to 2D h-BN coupled with substrate, while the higher energy sub-band is related to decoupled (or free-standing) 2D h-BN. It is further shown that this newly-observed fine structure in the A2u mode can be used to assess, quickly and easily, the homogeneity of the h-BN-metal interface and the effects of metal surface contamination on adhesion of the layer.