Fracture, delamination, and buckling of elastic thin films on compliant substrates

A series of studies have been conducted for mechanical behavior of elastic thin films on compliant substrates. Under tension, the film may fracture by growing channel cracks. The driving force for channel cracking (i.e., the energy release rate) increases significantly for compliant substrates. Moreover, channel cracking may be accompanied by interfacial delamination. For a film on a relatively compliant substrate, a critical interface toughness is predicted, which separates stable and unstable delamination. For a film on a relatively stiff substrate, however, a channel crack grows with no delamination when the interface toughness is greater than a critical value. An effective energy release rate for the steady-state growth of a channel crack is defined to account for the influence of inter-facial delamination on both the fracture driving force and the resistance, which can be significantly higher than the energy release rate assuming no delamination. Alternatively, when the film is under compression, it tends to buckle. Two buckling modes have been observed, one with interfacial delamination (i.e., buckle-delamination) and the other without delamination (i.e., wrinkling). By comparing the critical stresses for the onset of buckling, we give a criterion for the selection of the buckling modes, which depends on the stiffness ratio between the film and the substrate as well as the interface defects. A general conclusion from these studies is that, whether tension or compression, the interfacial properties are critical in controlling the morphology and failure of elastic thin films on compliant substrates.