Substrate-anisotropy-dominated bending fracture of hard-coated PET films for foldable display applications

  Chen Shen, Junhua Xiao, Guorui Wang*

Journal of Materials Science  (2026) doi: https://doi.org/10.1007/s10853-026-12197-x

ABSTRACT

   Foldable electronic devices require polymer cover films with reliable coating integrity under repeated bending. In this work, the bending-induced failure behavior of an acrylic hard coat layer on isotropic and anisotropic PET substrates is systematically investigated. Low-temperature inward bending tests reveal pronounced substrate- and orientation-dependent damage morphologies, including differences in crack opening, crack continuity, and damaged-zone width. Tensile characterization shows that substrate stiffness, strength, and ductility along the bending direction strongly affect the tolerance of coating to bending deformation. Wide angle X-ray scattering indicates that cyclic loading has little influence on overall crystallinity, whereas the stability of crystalline orientation varies significantly with substrate anisotropy. PET films with highly aligned and orientation-stable crystalline structures exhibit suppressed crack opening and improved resistance to bending-induced damage. By correlating coating fracture morphology with substrate mechanical properties and microstructural stability, this study demonstrates that bending durability of hard-coated PET films is primarily governed by substrate mechanical response and orientation stability rather than coating composition alone. These results provide practical guidelines for substrate selection and structural optimization of cover film systems in foldable display applications.