Mechanical failure of brittle thin films on polymers during bending by two-point rotation

• A 2-point Rotation (2PR) device was built to assess failure in thin brittle films.
• Electrical measurements were performed using the 2PR and 2-point Bending (2PB) method.
• The characteristic failure strain was determined by these two methods.
• The 2PR test yields the same accuracy as the conventional 2PB.
• Both tensile and compressive internal strain can be determined using the 2PR method.

The mechanical failure and internal strain when bending a brittle thin film on a flexible substrate was analyzed using an electro-mechanical method and optical microscopy. Bending was realized by clamping the ends of the sample and rotating the clamps in a two-point rotation (2PR) device, as elaborated in the paper. This test allows to impose both tensile and compressive strains on the brittle layer, in a single test, without remounting the sample. With the electro-mechanical method the real-time electrical resistance is measured in a very thin, conductive coating deposited on the brittle layer in which cracks develop across the width of the film. Simultaneously, the propagation of individual cracks is observed by optical microscopy. Real-time combination of both analyses in principle enables linking the fracture behavior (size, number and pattern of cracks) with the electrical resistance at a well-defined imposed strain.With this 2PR method, the subcritical mechanical failure of a 150 nm thick SiNx barrier layer was analyzed, as deposited on a 125 μm thick polyethylene naphtalate sheet (system 1). Its characteristic static failure time, as obtained from a Weibull analysis, was 0.10 s at a strain of 0.84% and 2.2 s at 0.77%. From the crack opening on reloading tests, an average internal compressive strain of 0.38% was deduced. In order to validate the results of the 2PR device, system 1 was also tested using two-plate bending (2PB), as described earlier. Results on characteristic failure strains as obtained with both methods for bending match very well and have similar accuracy. However, the option to assess tensile internal strains, together with the aforementioned advantages, renders the 2PR test the method of choice for testing brittle layers on polymer substrates. Comparison of system 1 with another barrier system (2), which only differs in processing conditions, indicates a considerable difference in time dependence of the characteristic failure strain, indicating the sensitivity of the resulting material properties on processing conditions.