On delamination growth from channel cracks in thin-film coatings

Interfacial fracture (delamination) originating from channel or tunnel cracks is a common failure mode in layered structures. While this subject has been addressed extensively, little is known on the actual process of fracture. It is this aspect which is of concern here. The evolution of delamination damage is followed in situ using an all-transparent system designed to reduce material variability and thermal stresses. The specimen is composed of two glass plates glued onto a polycarbonate slab by a RT epoxy resin. With a proper control of the glass surface, stable delamination growth from a single channel crack occurs. This growth evolves smoothly from the tip of the channel crack, although the delamination area is generally irregular and non-symmetric. The fracture resistance varies greatly between nominally identical samples, attesting to a great sensitivity to such irregularity as well as to bonding surface conditions. The effect of system variables on delamination growth is evaluated using a 2D FEA. The analysis predicts general trends observed in the tests, and it indicates means for an optimal design against premature failure. Finally, the merit of evaluating fracture toughness of ultra-thin films using the channel cracking approach is discussed.