E.G. Liniger

Bio: E.G. Liniger is an academic researcher from IBM. The author has contributed to research in topics: Delamination & Plane stress. The author has an hindex of 3, co-authored 3 publications receiving 401 citations.

Growth and configurational stability of circular, buckling-driven film delaminations

Abstract: A study is presented of delamination at the interface between a thin elastic film bonded to a substrate under conditions in which the film is subject to equi-biaxial compression. The focus is on initially circular delaminations. When the initial delamination is sufficiently large it buckles away from the substrate producing a blister which in turn induces a driving force on the interface crack tip. A two-part theoretical analysis of the coupled buckling/fracture problem is conducted: the axisymmetric growth of the circular blister, and the stability of the circular blister to nonaxisymmetric perturbations of the interface crack front. A simple criterion is identified which excludes the possibility of wide-spread delamination. Experiments are reported for a model film/substrate system (mica bonded to aluminum) chosen to allow visualization of the interface and to permit compressive stresses in the film to be generated over the full range of interest by exploiting the large thermal expansion mismatch of the system. The experiments bear out the theoretical prediction of a regime of axisymmetric growth which gives way to nonaxisymmetric blisters after a blister becomes sufficiently large. The study suggests that the wavy-circular and worm-like blister morphologies which are usually observed for delaminated films are a manifestation of the configurational instability of the interface crack front.

Plane-strain, buckling-driven delamination of thin films: Model experiments and mode-II fracture

Abstract: A series of model experiments have been conducted to compare observations on buckling-driven delamination of thin films under plane-strain compression with an existing analysis. The results are consistent within the range considered by the theory, which applies when the delamination crack is open to the tip. However, the observations indicate that delamination can occur beyond this range, when the crack tip is closed and undergoing mode-II advance. The theory was extended to incorporate the effect of a contact region in which frictional effects shields the crack tip. A comparison of the analysis with the data indicates that the frictional stress required to explain the apparent toughness observed in this regime is larger than the shear yield strength of the interface. It is inferred that large-scale plasticity may have a significant effect on the results, but this effect has not been included in the analysis.

Delamination from edge flaws

Abstract: The mechanics of delamination from an edge flaw in a laminated material is presented. Two different types of failure mechanism have been identified: uniform delamination along the crack front and localized delamination which can be initiated if the laminate buckles above the debonded portion of the interface. Depending on the stress state within the bonded portion of the laminate, this localized failure can initiate the formation of blisters which grow by a mechanism of buckling-driven delamination and can cause general failure of the laminate. There are conditions under which this class of buckling-assisted delamination occurs at loads lower than would be predicted for uniform delamination. It may, therefore, have important implications in the design against failure for thin films, composites and other laminated systems. Experimental observations of the different failure mechanisms are presented and the factors governing the transitions between them are discussed.

Spontaneous formation of ordered structures in thin films of metals supported on an elastomeric polymer

Abstract: Spontaneous generation of complex order in apparently simple systems is both arresting and potentially useful1,2,3,4,5,6,7,8,9,10,11. Here we describe the appearance of complex, ordered structures induced by the buckling of thin metal films owing to thermal contraction of an underlying substrate. We deposit the films from the vapour phase on a thermally expanded polymer (polydimethylsiloxane, PDMS). Subsequent cooling of the polymer creates compressive stress in the metal film that is relieved by buckling with a uniform wavelength of 20–50 micrometres. The waves can be controlled and orientated by relief structures in the surface of the polymer, which can set up intricate, ordered patterns over large areas. We can account qualitatively for the size and form of the patterned features in terms of the non-uniform stresses developed in the film near steps on the polymer substrate. This patterning process may find applications in optical devices such as diffraction gratings and optical sensors, and as the basis for methods of strain analysis in materials.

Thin Film Materials: Stress, Defect Formation and Surface Evolution

Abstract: 1. Introduction and overview 2. Film stress and substrate curvature 3. Stress in anisotropic and patterned films 4. Delamination and fracture 5. Film buckling, bulging and peeling 6. Dislocation formation in epitaxial systems 7. Dislocation interactions and strain relaxation 8. Equilibrium and stability of surfaces 9. The role of stress in mass transport.

Harnessing Surface Wrinkle Patterns in Soft Matter

Abstract: Mechanical instabilities in soft materials, specifically wrinkling, have led to the formation of unique surface patterns for a wide range of applications that are related to surface topography and its dynamic tuning. In this progress report, two distinct approaches for wrinkle formation, including mechanical stretching/releasing of oxide/PDMS bilayers and swelling of hydrogel films confined on a rigid substrate with a depth-wise modulus gradient, are discussed. The wrinkling mechanisms and transitions between different wrinkle patterns are studied. Strategies to control the wrinkle pattern order and characteristic wavelength are suggested, and some efforts in harnessing topographic tunability in elastomeric PDMS bilayer wrinkled films for various applications, including tunable adhesion, wetting, microfluidics, and microlens arrays, are highlighted. The report concludes with perspectives on the future directions in manipulation of pattern formation for complex structures, and potential new technological applications.

Stiction in surface micromachining

Abstract: Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces.