Epoxy-Functionalized Polysilsesquioxane Coatings for Rear-Side Impact Protection of Thin Functional Ceramics

High-performance protective coatings capable of preventing catastrophic impact fracture of thin brittle inorganic substrates are urgently demanded for next-generation 5G/6G consumer electronics; however rational design strategies that simultaneously retain pristine surface aesthetics and achieve prominent shock resistance are still lacking. Herein, a rigid epoxy-functionalized polysilsesquioxane hybrid resin is synthesized, followed by cross-linking with various amine curing agents through a high-efficiency synchronous thermal-moisture dual-curing route. The optimized EGSI coating crosslinked by isophorone diamine (IPDA) achieves a pencil scratch hardness of 9H and a nanoindentation modulus of 2.66 GPa. Viscoelastic characterization confirms a distinctive filler-free self-reinforcement effect originating from the siloxane skeleton; the glass transition becomes indistinct, and the optimized coating preserved ∼40% of its initial glassy storage modulus, corresponding to a remarkably high rubbery plateau modulus of ∼750 MPa up to 300 °C. Ball-drop impact tests support the effectiveness of the proposed rigid backing structural constraint mechanism. When coated on the rear face of 300 μm structural ceramic sheets, the EGSI layer elevates the maximum crack-free nominal drop energy to 63.9 mJ, representing a 233% enhancement compared with bare ceramics. Different from conventional front-side energy-absorbing coatings, this noninvasive rear-side reinforcement scheme avoids damaging the decorative exterior of ceramic components and enables low-cost upgrading of ordinary sintered ceramics to high shock-resistant grade. This study establishes a design approach for the tested systems and coating configuration optimization methodology for reinforcing brittle substrates, showing great application prospects in smartphone ceramic back covers, ultrathin glass panels, and other electronic protective components.