Biaxial fragmentation of thin silicon oxide coatings on poly(ethylene terephthalate)

Crack patterns of 53 nm and 103 nm thick silicon oxide coatings on poly(eth ylene terephthalate) films are analyzed under equibiaxial stress loading, b y means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The biaxial stress and strain are modele d from classical elastic membrane equations, and an excellent agreement is obtained with a finite element method. In the large pressure range, the der ivation of the biaxial strain from force equilibrium considerations are fou nd to reproduce accurately the measured data up to 25% strain. The examinat ion of the fragmentation process of the coating under increasing pressure l evels reveals that the crack onset strain of the oxide coating is similar t o that measured under uniaxial tension. The fragmentation of the coating un der biaxial tension is also characterized by complex dynamic phenomena whic h image the peculiarities of the stress field, resulting in considerable br oadening of the fragment size distribution. The evolution of the average fr agment area as a function of biaxial stress in the early stages of the frag mentation process is analyzed using Weibull statistics to describe the coat ing strength. (C) 2001 Kluwer Academic Publishers.