Bauschinger effect in thin metal films on compliant substrates

Flexible electronic devices such as flexible displays and solar cells draw more and more attention from the industry. In these devices, the functionality is delivered by small-scale structures consisting of thin metal lines and other materials that are integrated on a compliant (e.g. polymer) substrate to make the device flexible. A major concern is reliability, since the small-scale structures with features on the order of micrometers or below are highly fragile, however, they are subjected to large thermo-mechanical loads during manufacturing and use. Although a growing literature exists on rupture and buckling of thin metal films on polymer substrates, see for instance [1], many mechanical issues of flexible electronics have not been studied in depth. In this project, one such issue, the Bauschinger effect of thin metal films bonded to a compliant substrate, will be studied in more detail, because the Bauschinger effect appears to be a particularly important issue in flexible electronics applications where the device is subjected to multiple strain cycles

The general 3D Hertzian contact problem for anisotropic materials

This paper presents a general method for solving the 3D frictionless contact problem between generally anisotropic materials with any second order surface geometry. The method uses the Stroh formalism to find the Green's Functions (GF) of the materials with an efficient numerical integration process. The GFs are then expanded in Fourier series in order to solve the Hertzian contact problem between the two bodies as a perturbation to the first order, 2equivante isotropic2, solution to the problem. The latter permits to define an 2equivalent indentation modulus of the contact" which is a single parameter computed from the first terms of the Fourier expansion of the two GFs (ie the average values) and permits to use the standard Hertzian solution: this gives a good approximation to the contact area (at most elliptical in any case) which is approximated as a circle for axi-symmetrical geometry, and for the approach of remote points in the two bodies. The "equivalent indentation modulus", which depends on materials and orientation, is computed for a set of composite materials of practical interest

Bauschinger effect in thin metal films on compliant substrates

Flexible electronic devices such as flexible displays and solar cells draw more and more attention from the industry. In these devices, the functionality is delivered by small-scale structures consisting of thin metal lines and other materials that are integrated on a compliant (e.g. polymer) substrate to make the device flexible. A major concern is reliability, since the small-scale structures with features on the order of micrometers or below are highly fragile, however, they are subjected to large thermo-mechanical loads during manufacturing and use. Although a growing literature exists on rupture and buckling of thin metal films on polymer substrates, see for instance [1], many mechanical issues of flexible electronics have not been studied in depth. In this project, one such issue, the Bauschinger effect of thin metal films bonded to a compliant substrate, will be studied in more detail, because the Bauschinger effect appears to be a particularly important issue in flexible electronics applications where the device is subjected to multiple strain cycles

Laser annealing of amorphous NiTi shape memory alloy thin films to locally induce shape memory properties

We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation

Thermal modeling of laser-annealing-induced crystallization of amorphous NiTi thin films

Laser annealing of shape memory alloy thin films provides new opportunities in actuator design and fabrication for microelectromechanical systems applications. In this paper, we present a three-dimensional thermal model to simulate the crystallization process when a laser beam is swept across an amorphous NiTi thin film. Experimental crystallite nucleation and growth rates are included in the model to enable prediction of the size of the crystallized region as a function of laser annealing parameters. The model can also be used to study the crystallization of other material systems by means of laser annealing

The indentation modulus of elastically anisotropic materials for indenters of arbitrary shape

The contact of an indenter of arbitrary shape on an elastically anisotropic half space is considered. It is demonstrated in a theorem that the solution of the contact problem is the one that maximizes the load on the indenter for a given indentation depth. The theorem can be used to derive the best approximate solution in the Rayleigh–Ritz sense if the contact area is a priori assumed to have a certain shape. This approach is used to analyze the contact of a sphere and an axisymmetric cone on an anisotropic half space. The contact area is assumed to be elliptical, which is exact for the sphere and an approximation for the cone. It is further shown that the contact area is exactly elliptical even for conical indenters when a limited class of Green's functions is considered. If only the first term of the surface Green's function Fourier expansion is retained in the solution of the axisymmetric contact problem, a simpler solution is obtained, referred to as the equivalent isotropic solution. For most anisotropic materials, the contact stiffness determined using this approach is very close to the value obtained for both conical and spherical indenters by means of the theorem. Therefore, it is suggested that the equivalent isotropic solution provides a quick and efficient estimate for quantities such as the elastic compliance or stiffness of the contact. The "equivalent indentation modulus", which depends on material and orientation, is computed for sapphire and diamond single crystals

Reprint of: Nanocalorimetry: Door opened for in situ material characterization under extreme non-equilibrium conditions

The past two decades have witnessed the rapid development of nanocalorimetry, a novel materials characterization technique that employs micromachined calorimetric sensors. The key advances of this technique are the ultrahigh scanning rate, which can be as high as 106 K/s, and the ultrahigh heat capacity sensitivity, with a resolution typically better than 1 nJ/K. Nanocalorimetry has attracted extensive attention in the field of materials science, where it is applied to perform quantitative analysis of rapid phase transitions. This paper reviews the development of nanocalorimetry over the last three decades and summarizes its applications to various materials ranging from polymers to metals. The glass transition and crystallization of non-crystalline materials, melting and solidification of metallic droplets, and solid-state phase transitions of thin films are introduced as typical examples. Furthermore, nanocalorimetry coupled with structural characterization techniques, such as transmission electron microscopy and synchrotron X-ray diffraction, is presented. Finally, current challenges and future outlooks for the technique are discussed. Given the unique attributes of the technique, we expect nanocalorimetry to attract increasing attention, especially with regard to characterization of fast phase transitions and evaluation of size effects

Laser annealing of amorphous ni-ti shape memory alloy thin films

Shape memory alloy (SMA) thin films, especially those based on Ni-Ti, have received considerable attention since it was demonstrated that they can undergo a nearly perfect shape memory effect. When SMA films are used as actuators in MEMS devices, biasing springs are generally needed to restore the initial state in order to achieve a two-way shape memory effect. As a result, use of SMA actuator in MEMS has been limited mainly to bimorph-like mechanisms. Recently, laser annealing of shape memory alloys (LASMA) emerged as a promising approach for the fabrication of planar mechanisms. This technique has the advantage that shape memory properties can be spatially distributed: material crystallized by laser irradiation has shape memory properties and can be used as an actuator, while untransformed material is passive and provides a restoring force. In this paper, we present the results of an experimental study of the laser annealing process for NiTi thin films, along with a computational model

Dislocation climb in two-dimensional discrete dislocation dynamics

In this paper, dislocation climb is incorporated in a two-dimensional discrete dislocation dynamics model. Calculations are carried out for polycrystalline thin films, passivated on one or both surfaces. Climb allows dislocations to escape from dislocation pile-ups and reduces the strain-hardening rate, especially for fully passivated films. Within the framework of this model, climb modifies the dislocation structures that develop during plastic deformation and results in the formation of pile-ups on slip planes that do not contain any dislocation sources.Materials Science and EngineeringMechanical, Maritime and Materials Engineerin