Analysis of the initial fragmentation stage of oxide coatings on polymer substrates under biaxial tension

Crack patterns of 100-nm thick silicon oxide coatings on poly(ethylene terephthalate), polypropylene and polyamide films are analyzed under equibiaxial stress loading. The loading is achieved by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The evolution of the coating fragment area distribution in the early stages of the fragmentation process is modeled using Weibull statistics to describe the coating strength. Fragment areas are found to follow the exponential distribution at low strains close to the cracking onset. With the increase of equibiaxial strain, the distribution changes reflecting stress transfer and crack propagation features.