Nonlinear delamination mechanics for thin films

Delamination of prestressed thin films on thick substrates is analysed accounting for plastic dissipation in either the substrate or film. Emphasis is on large scale yielding wherein the height of the plastic zone at the propagating interface crack tip is comparable to the film thickness. Such conditions are common for both metal and polymer thin films on elastic substrates or for ceramic coatings on metal substrates when the interface between the film and substrate is reasonably strong. Under large scale yielding, the notion of a thickness-independent interface toughness no longer pertains, and a nonlinear fracture mechanics is required to quantify delamination. Two such approaches are pursued in this paper using models based on the attainment of critical conditions at the interface crack tip within the plastic zone. Steady-state film delamination is analysed for conditions where yielding occurs either in the film or in the substrate, and critical combinations of prestress and thickness are predicted. The theory is applied to a recent set of experiments on copper films delaminating from silica substrates.