The indentation behavior of cold-rolled AA6061 Al alloy was investigated. Following the approach suggested by Tabor, indentation stress–indentation strain curves were constructed and analyzed. The indentation stress required to create the same indentation strain increases with an increase in the reduction of thickness, suggesting a strong effect of plastic deformation history on the deformation behavior of materials. Through the dislocation dynamics, the evolution of the dislocations underneath the indentation was correlated with the plastic deformation history and the indentation load. The plastic energy dissipated in indentation was then calculated and found to be proportional to the 3/2 power of the indentation load and the 3/4 power of the average dislocation density underneath the indentation. The ratio of the dissipated plastic energy to the total energy in the indentation was demonstrated to be a function of the deformation state in materials, independent of the indentation load.

The Lithographie, Galvanoformung, Abformung (LIGA) technique is important for making metal-based high-aspect-ratio microscale structures (HARMS) and microdevices derived from metal-based HARMS. Recently, molding replication of HARMS made of Pb, Zn, and Al has been demonstrated, advancing LIGA technology from the state where only polymer-based HARMS could be replicated by molding. This demonstration offers a potential means for economical fabrication of a wide variety of metal-based microdevices. Micromolding of a metal requires heating the metal to be molded to a significant fraction of its melting temperature. At high temperatures, the strength of the mold insert itself will typically decrease. The insert strength thus places a limit on the range of materials that can be molded. In this paper, micromolding and tensile experiments on Pb were carried out. A simple mechanics model of the micromolding process was developed. This model relates the stresses on the insert during micromolding primarily to the yield strength of the molded metal and frictional tractions on the sides of the insert. Reasonable agreement was obtained between the Pb experiments and the model predictions. Ramifications for other material systems are discussed.

Nix and Gao established an important relation between microindentation hardnessand indentation depth. Such a relation has been verified by many microindentation experiments (indentation depths in the micrometer range), but it does not always hold in nanoindentation experiments (indentation depths approaching the nanometer range). We have developed a unified computational model for both micro- and nanoindentation in an effort to understand the breakdown of the Nix–Gao relation at indentation depths approaching the nanometer scale. The unified computational model for indentation accounts for various indenter shapes, including a sharp, conical indenter, a spherical indenter, and a conical indenter with a spherical tip. It is based on the conventional theory of mechanism-based strain gradient plasticity established from the Taylor dislocation model to account for the effect of geometrically necessary dislocations. The unified computational model for indentation indeed shows that the Nix–Gao relation holds in microindentation with a sharp indenter, but it does not hold in nanoindentation due to the indenter tip radius effect.

In this paper, a new scheme has been proposed for the computational modeling of conical indentation in plastically graded materials. Based on the new scheme and the work of Dao et al. [Acta Mater.49, 3899 (2001)], and taking the example of conical indentation with an indenter whose tip apex angle is θ = 70.3°, an analytical expression to predict the loading P - h curve for the indentation of plastically graded materials has been presented. A reverse algorithm has further been established to determine the plastic properties of a plastically graded surface. The existence, uniqueness, and stability of the solution to the inverse problem have been systematically investigated. The current work can be applied to the evaluation or optimization of various plastically graded surfaces.

Silicon is a principal material in submicrometer-scale devices. Components in such devices are subject to intense local stress concentrations from nanoscale contacts during function. Questions arise as to the fundamental nature and extent of any strength-degrading damage incurred at such contacts on otherwise pristine surfaces. Here, a simple bilayer test procedure is adapted to probe the strengths of selected areas of silicon surfaces after nanoindentation with a Berkovich diamond. Analogous tests on silicate glass surfaces are used as a control. The strengths increase with diminishing contact penetration in both materials, even below thresholds for visible cracking at the impression corners. However, the strength levels in the subthreshold region are much lower in the silicon, indicating exceptionally high brittleness and vulnerability to small-scale damage in this material. The results have important implications in the design of devices with silicon components.

Finite element simulation of the Berkovich, Vickers, Knoop, and cone indenters was carried out for the indentation of elastic–plastic material. To fix the semiapex angle of the cone, several rules of equivalence were used and examined. Despite the asymmetry and differences in the stress and strain fields, it was established that for the Berkovich and Vickers indenters, the load–displacement relation can closely be simulated by a single cone indenter having a semiapex angle equal to 70.3° in accordance with the rule of the volume equivalence. On the other hand, none of the rules is applicable to the Knoop indenter owing to its great asymmetry. The finite element method developed here is also applicable to layered or gradient materials with slight modifications.

Depth-sensing indentation at ultramicroscopic and macroscopic contacts (“nanoindentation” and “macroindentation,” respectively) was performed on four brittle materials (soda-lime glass, alumina titanium carbide, sapphire, and silicon) and the resulting load–displacement traces examined to provide insight to the elastic and plastic deformation scaling with contact size. The load–displacement traces are examined in terms of the unloading stiffness, the energies deposited during loading and recovered on unloading, and the effect of the indenter tip radius on the loading curve. The results of the analyses show that the elastic and plastic deformation during loading and unloading is invariant with the scale of the contact, and the unloading curve is best described by neither a conical tip nor a paraboloid of revolution, but of some compromise.

Electron beam physical-vapor-deposited Y2O3-stabilized ZrO2 thermal barrier coating (TBC) samples were indented from room temperature to 900 °C using an instrumented high-temperature vacuum displacement-sensitive indenter. Hardness and elastic modulus were determined from the load–displacement curves recorded during indentation. Both the hardness and the elastic modulus of the TBCs were much lower than those of dense ceramics of a similar composition; this is attributed to the increased compliance that results from the porous columnar microstructure of the TBCs. In addition, the TBCs showed an unusual absence of elastic recovery at the residual indents compared to the dense ceramics.

The morphology and distribution of high-pressure metastable phases BC8 and R8, formed in monocrystalline silicon under microindentation, were identified and assessed using transmission electron microscopy nanodiffraction analysis. It was discovered that the crystal growth inside the transformation zone was stress-dependent with large crystals in its central region. The crystal size could also be increased using higher maximum indentation loads. The BC8 and R8 phases distributed unevenly across the transformation zone, with BC8 crystals mainly in the center of the zone and smaller R8 fragments in the peripheral regions. Such phase distribution was in agreement with the theoretical residual stress analysis.