Experimental assessment of micromechanical models for fragmentation analysis of thin metal oxide coatings on polymer films under uniaxial tensile deformation

The barrier properties of metal oxide coatings deposited on polymer substrates is of interest in packaging applications for beverages and food. The durability of the coatings is compromised mechanically due to brittleness of the coatings, leading to multiple cracking when deformed. This cracking behavior has been investigated in situ for atomic layer deposited coatings of TiO2 (6 and 20 nm thick) and mixed oxides of TiO2 and Al2O3 (4 and 20 nm thick) on polyethylene terephthalate substrate films by observing multiple cracking under uniaxial tension. Two key models to identify cohesive and adhesive properties from the fragmentation data have been implemented, and their differences are discussed in light of the new experimental data. Such material properties include the Weibull strength distribution of the coating, the interfacial shear strength, the strength at crack saturation, and fracture toughness at crack onset and at saturation. These properties can be useful for materials selection and possibly for design simulations, when the unaccounted effects like finite deformation and substrate yielding do not take over. A significantly weaker interface and cohesive strength was observed for the thinner mixed-oxide coating, where the dimensions of the material heterogeneities were of the same order as the coating thickness. These dimensions seem to be the limiting factor of how thin coatings can be made without a major loss in structural integrity.