Mechanical characterization and modelling of the fracture behaviour of thin brittle coatings on polymer films

Abstract: Thin brittle coatings with thickness of a few nanometres deposited on flexible polymer substrates form an interesting material combination for food packaging and flexible electronics. This combination may enhance the barrier performance in carton packages. In flexible electronics, coatings could conduct electricity despite their small thickness. However, cracking is a concern when brittle coatings are subjected to tension and bending in the manufacturing process or in final products with curved shapes. Therefore, the fracture behavior of potential coatings for such applications was studied by a combined use of experimental, analytical and finite element methods.In the experimental work, the input parameters for nanometre-scale models were quantified. These parameters represent properties of the coating and of the interface between substrate and coating. The Young’s moduli of the coatings were estimated by use of three independent experimental methods: nanoindentation, buckling of coatings, and atomic force microscopy (AFM). The advantages and drawbacks of these methods were addressed for each material system. Then, multiple cracking of coatings subjected to uniaxial tensile loading was examined in situ by scanning electron microscopy using high magnification and resolution. The strain fields of the coated substrates were measured up to large tensile deformations by use of digital image correlation. Statistical micromechanical models were adapted and used for the multiple cracking behaviour of the coatings. Adhesive and cohesive properties, such as interfacial shear strength and coating strength, were determined. The multiple cracking behaviour was investigated also by using linear fracture mechanics and finite elements models to obtain energy release rates. The interfacial energy release rates were quantified also from local delamination of the coatings due transverse Poisson contraction of the substrates loaded in tension. Nanoindentation turned out to be a useful technique for measuring the Young’s modulus of coatings. Suitable indentation depth was found to be around 10% of the coating thickness. AFM was found to be the best method because it was found to minimize the viscous effect of the polymer. Because of delamination, buckling of coatings was not suitable for estimation of the Young’s modulus. However, ensuing ridged cracks were suitable to estimate interfacial energy release rates. The progressive crack density was examined in detail, and in high magnification details of the cracks were observed, such as the crack-opening displacements needed for prediction of barrier properties. Measured material properties on nanometre are useful for industrial implementation of coating technology and selecting coating materials for sustainable product design.