Yang-Tse Cheng

Relationships between hardness, elastic modulus, and the work of indentation

The work done during indentation is examined using dimensional analysis and finite element calculations for conical indentation in elastic-plastic solids with work hardening. An approximate relationship between the ratio of hardness to elastic modulus and the ratio of irreversible work to total work in indentation is found. Consequently, the ratio of hardness to elastic modulus may be obtained directly from measuring the work of indentation. Together with a well-known relationship between elastic modulus, initial unloading slope, and contact area, a new method is then suggested for estimating the hardness and modulus of solids using instrumented indentation with conical or pyramidal indenters.

When is thermodynamics relevant to ion-induced atomic rearrangements in metals?

Abstract The problem of ion-induced mixing of metal bilayers is examined in the limit of heavy metals (Z ≳ 20) and heavy energetic ions (E ≳ 100 keV) and in the absence of delayed effects such as radiation enhanced thermal diffusion. Thermochemical effects are shown to play an important role in biasing the random walk process of mixing. A universal mixing equation is derived which predicts the evolution of the concentration profile as a function of ion dose. Finally, a model is presented which allows one to predict what metallurgical phases are formed during the mixing process. Criteria for amorphous phase formation are particularly emphasized.

Is the lotus leaf superhydrophobic

Superhydrophobic surfaces have important technical applications ranging from self-cleaning window glasses, paints, and fabrics to low-friction surfaces. The archetype superhydrophobic surface is that of the lotus leaf. When rain falls on lotus leaves, water beads up with a contact angle in the superhydrophobic range of about 160°. The water drops promptly roll off the leaves collecting dirt along the way. This lotus effect has, in recent years, stimulated much research effort worldwide in the fabrication of surfaces with superhydrophobicity. But, is the lotus surface truly superhydrophobic? This work shows that the lotus leaves can be either hydrophobic or hydrophilic, depending on how the water gets on to their surfaces. This finding has significant ramifications on how to make and use superhydrophobic surfaces.

Scaling approach to conical indentation in elastic-plastic solids with work hardening

We derive, using dimensional analysis and finite element calculations, several scaling relationships for conical indentation in elastic-plastic solids with work hardening. Using these scaling relationships, we examine the relationships between hardness, contact area, initial unloading slope, and mechanical properties of solids. The scaling relationships also provide new insights into the shape of indentation curves and form the basis for understanding indentation measurements, including nano- and micro-indentation techniques. They may also be helpful as a guide to numerical and finite element calculations of indentation problems.

Effects of micro-?and nano-structures on the self-cleaning behaviour of lotus leaves

When rain falls on lotus leaves water beads up with a high contact angle. The water drops promptly roll off the leaves, collecting dirt along the way. This self-cleaning ability or lotus effect has, in recent years, stimulated much research effort worldwide for a variety of applications ranging from self-cleaning window glasses, paints, and fabrics to low friction surfaces. What are the mechanisms giving rise to the lotus effect? Although chemical composition and surface structure are believed important, a systematic experimental investigation of their effects is still lacking. By altering the surface structure of the leaves while keeping their chemical composition approximately the same, we report in this study the influence of micro- and nano-scale structures on the wetting behaviour of lotus leaves. The findings of this work may help design self-cleaning surfaces and improve our understanding of wetting mechanisms.

Thermodynamic and fractal geometric aspects of ion-solid interactions

A thermodynamic approach to atomic diffusion in a thermal spike is reviewed. The approach is based on recent ion mixing experiments which demonstrate the influence of the heat of mixing and the cohesive energy of solids on ion mixing. These thermodynamic effects are assimilated into a phenomenological model of ion mixing. The model is generalized to low-energy ion mixing during sputter depth profiling and is used to elucidate the nature of atomic diffusion in a thermal spike. The onset of radiation-enhanced diffusion in ion mixing is also discussed. A fractal geometry approach to spike formation is presented. An “idealized” collision cascade constructed from the inverse-power potential V ( r ) ∝ r −1/ m (0 m ≤ 1) is shown to have a fractal tree structure with a fractal dimension D = 1/2 m . The same fractal dimension can also be derived from the Winterbon-Sigmund-Sanders (WSS) theory of atomic collisions in solids. The fractal dimension is shown to increase as an actual collision cascade evolves, because of the change of the effective interaction potentials. The concept of “space-filling” fractals is used to specify spikes. The formation of local spikes, their energy densities, the probability of local spikes overlapping, and the time evolution of a collision cascade are also investigated. It is shown that spikes are not expected to form in a single-component solid consisting of elements with atomic number less than 20; many-body collisions have little effect on the formation of spikes; and, the similarity between high-and low-energy ion mixing is the result of the fractal nature of collision cascades.

The influence of surface mechanics on diffusion induced stresses within spherical nanoparticles

We examine the effects of surface tension and surface modulus on diffusion-induced stresses within spherical nanoparticles. We show that both the magnitude and distribution of stresses can be significantly affected by surface mechanics if the particle diameter is in the nanometer range. In particular, a tensile state of stress may be significantly reduced in magnitude or even be reverted to a state of compressive stress with decreasing particle radius. This reduction in tensile stress may be responsible for the observed resilience to fracture and decrepitation of nanoparticles used in various industrial applications.

Scaling relationships in conical indentation of elastic-perfectly plastic solids

Using dimensional analysis and finite element calculations we derive several scaling relationships for conical indentation into elastic-perfectly plastic solids. These scaling relationships provide new insights into the shape of indentation curves and form the basis for understanding indentation measurements, including nano- and micro-indentation techniques. They are also helpful as a guide to numerical and finite element calculations of conical indentation problems. Finally, the scaling relationships are used to reveal the general relationships between hardness, contact area, initial unloading slope, and mechanical properties of solids.

Revealing Triple‐Shape Memory Effect by Polymer Bilayers

Bilayer polymers that consist of two epoxy dual-shape memory polymers of well-separated glass transition temperatures have been synthesized. These bilayer epoxy samples exhibit a triple-shape memory effect (TSME) with shape fixities tailorable by changing the ratio between the two layers. The triple-shape fixities of the bilayer epoxy polymers can be explained by the balance of stress between the two layers. Based on this work, it is believed that the following three molecular design criterions should be considered in designing triple-shape memory polymers with optimum TSME: 1) well-separated thermal transitions, 2) a strong interface, and 3) an appropriate balance of moduli and relative ratios between the layers (or microphases).

Diffusion-Induced Stress, Interfacial Charge Transfer, and Criteria for Avoiding Crack Initiation of Electrode Particles

Most lithium-ion battery electrodes experience large volume changes caused by concentration changes within the host particles during charging and discharging. Electrode failure, in the form of fracture or decrepitation, can occur as a result of repeated volume changes. In this work, we first develop analytic solutions for the evolution of concentration and stresses within a spherical electrode element under charging-discharging conditions when the system thermodynamics are ideal (e.g., no repulsion forces are significant between intercalate species). Both interfacial (electrochemical) kinetics and intercalate diffusion are comprehended. Based on the analytic solutions, we propose tensile stress-based criteria for the initiation of cracks within a spherical insertion electrode. These criteria may help guide the development of new materials for lithium-ion batteries with enhanced mechanical durability and identify battery operating conditions that, when maintained, keep the mechanical stresses below acceptable values, thereby increasing cell life.