Strain-rate sensitivity of hardness of nanocrystalline Ni75at.%Al25at.% alloy film

Room-temperature indentation experiments carried out on nanocrystalline Ni75at.%Al25at.% alloy films with a range of grain sizes revealed that the strain-rate sensitivity of hardness is nearly zero and that the hardness increases as grain size decreases. The strain-rate insensitivity of hardness indicates that the room-temperature strength of these alloy films is dominated by an athermal, strain-rate-insensitive component. The hardness of the films was found to be in the range of 2.4 to 3.3 GPa, depending on grain size.published_or_final_versio

Materials Characterization Using High-Frequency Atomic Force Microscopy and Friction Force Microscopy

During the last decade, Atomic Force Microscopy (AFM) has been widely used to image the topography of various surfaces with corrugations down to the atomic scale [1,2]. Since then, development of new techniques based on AFM has been conducted to evaluate physical, chemical or mechanical surface properties [3]. We describe the use of near-field acoustic microscopy, based on AFM and hereafter referred to as Acoustic Microscopy by Atomic Force Microscopy (AFAM), as it has been developed earlier [4]. The relevance of this new scanning probe microscopy for high-resolution nondestructive testing and evaluation purposes is pointed out. It is shown that AFAM is capable of measuring elasticity on surfaces with a spatial resolution of less than 100 nm. Subsurface elastic properties and subsurface microdefect characterization can be performed by this technique. The high frequency Friction Force Microscopy (FFM) image, hereafter called Acoustic Friction Force Micropscopy (AFFM), reveals information different from the conventionally taken friction force image. We describe experimental and theoretical aspects of high-frequency atomic force and friction force microscopy

Nanoscratch Characterization of GaN Epilayers on c- and a-Axis Sapphire Substrates

In this study, we used metal organic chemical vapor deposition to form gallium nitride (GaN) epilayers on c- and a-axis sapphire substrates and then used the nanoscratch technique and atomic force microscopy (AFM) to determine the nanotribological behavior and deformation characteristics of the GaN epilayers, respectively. The AFM morphological studies revealed that pile-up phenomena occurred on both sides of the scratches formed on the GaN epilayers. It is suggested that cracking dominates in the case of GaN epilayers while ploughing during the process of scratching; the appearances of the scratched surfaces were significantly different for the GaN epilayers on the c- and a-axis sapphire substrates. In addition, compared to the c-axis substrate, we obtained higher values of the coefficient of friction (μ) and deeper penetration of the scratches on the GaN a-axis sapphire sample when we set the ramped force at 4,000 μN. This discrepancy suggests that GaN epilayers grown on c-axis sapphire have higher shear resistances than those formed on a-axis sapphire. The occurrence of pile-up events indicates that the generation and motion of individual dislocation, which we measured under the sites of critical brittle transitions of the scratch track, resulted in ductile and/or brittle properties as a result of the deformed and strain-hardened lattice structure

The insulator uncovered.

Microscopes that reveal the structures of surfaces one atom at a time are good at imaging conductors but perform poorly with insulators. That may be about to change

A density functional study of Br on Cu(100) at low coverages

Density functional theory has been used to study the adsorption of Br on Cu(1 0 0) at low coverages. As expected, the most stable binding site for Br atoms is the fourfold hollow site. The barrier for Br hopping between adjacent hollow sites along 〈0 1 1〉 directions via twofold saddle points is estimated to be 0.23 eV, whereas the barrier for hopping along 〈0 1 0〉 directions via an on-top site is 0.37 eV. The low barriers for motion between hollow sites indicate that it should not be possible to image low coverages of Br on Cu(1 0 0) at room temperature in scanning tunnelling microscopy. It is suggested that features previously attributed to Br atoms in hollow sites should in fact be assigned to Br bound in surface copper vacancy sites. © 2002 Elsevier Science B.V. All rights reserved