Thickness-dependent fracture behaviour of flexible ZnO : Al thin films

The effects of thickness on flexibility and crack initiation in ZnO : Al thin films sputter-deposited on polyethersulfone substrates have been investigated. With an increase in thickness, root-mean-square roughness and average crystallite size increase linearly. It is found that the higher the thickness, the lower is the strain required to initiate cracks in the film. The thinnest film (∼240 nm) exhibits a crack-initiating critical strain of 0.96% and a saturated crack density of 0.10 µm−1. A critical energy release rate of 68.5 J m−2 and a mode I fracture toughness of 3.2 MPa m0.5 are estimated for the films. These parameters are found to exhibit a linear dependence on film thickness.