Fatigue life study of ITO/PET specimens in cyclic bending tests

A commercial micro-compression tester and a self-designed bending tester in reciprocating motion are adopted to study the fatigue resistance and thus fatigue life of indium tin oxide (ITO) films deposited on a poly(ethylene terephthalate) substrate with various prestrain levels. The fatigue life obtained from tests reflects the size-dependent fatigue behavior. Two points symmetric with respect to the specimen central line are marked on the specimens of these two testers in order to measure the film’s electrical resistance variation with the number of cycles in the bending tests. The fatigue life is obtained as the number of cycles in the bending test based on the number of cycles corresponding to a 63 % failure strain of the specimen. The electrical resistance change in a test with a sufficiently large number of cycles increases nonlinearly with the compression force. The mean value of the total lengths of microcracks formed at fixed locations in the ITO film is identified as a major factor in the electrical resistance rise (ΔR). The (622) peak intensity of the X-ray diffraction patterns slightly affects ΔR. The combined effect of the microcrack length and (622) peak intensity can explain the ΔR trend for the specimens with different substrate prestrain. The fatigue lives predicted using a 10 % rise in the initial electrical resistance and a 63 % Weibull strain percentile are compared and discussed.