In situ nanoscale in-plane deformation studies of ultrathin polymeric films during tensile deformation using atomic force microscopy and digital image correlation techniques

The local, nanoscale deformation behavior of ultrathin polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films used as substrates in magnetic tapes was studied by atomic force microscopy (AFM) and digital image correlation (DIC) techniques. A custom-designed tensile stage was integrated with the AFM to perform uniaxial tension tests on the polymeric films in situ where the film surfaces were imaged simultaneously by AFM. The surface features on the PET and PEN films were used as reference patterns for the DIC processing. To improve the accuracy of the AFM imaging system for the application of the DIC method, a simple, cost-effective experimental procedure was established. Axial and transverse strain fields and Poisson's ratio maps with a spatial resolution of 78.13 nm were constructed via processing the AFM images of unstretched and stretched samples with the DIC software. Results from the AFM studies indicate that the deformation in both PET and PEN is nonuniform at the nanoscale. The nanoscale deformation mechanisms are discussed in conjunction with the structure of the PET and PEN films.