Performance of Multi-Gradient Biomimetic POSS-Toughened CFRP Laminates with a Pearl-Inspired Hard-Soft Interfacial...

Carbon-fiber-reinforced epoxy (CFRP) composites are widely used in aerospace structures because of their high specific strength and modulus; however, matrix cracking and interlaminar delamination induced by low-velocity impact compromise their structural safety. To overcome the difficulty of simultaneously improving interfacial strength and impact resistance through uniform nanomodification, a pearl-inspired gradient interfacial architecture incorporating a through-thickness distribution of polyhedral oligomeric silsesquioxane (POSS) was developed and compared with unmodified and uniformly modified CFRP laminates. SEM/EDS, TEM, double-cantilever-beam testing, low-velocity impact testing, compression-after-impact (CAI) testing, and micro-CT were combined to examine the effects of POSS spatial distribution on interfacial morphology, Mode I interlaminar fracture toughness, and impact-damage evolution. POSS incorporation increased interfacial roughness and produced nanoscale POSS-rich domains within the epoxy matrix. Although uniform addition of 0.5 wt% POSS increased the mean CAI strength to 142 MPa, the Mode I interlaminar fracture toughness decreased to 0.479 kJ·m^-2, revealing a trade-off in performance. By contrast, the gradient interface combined a high-POSS region with low-POSS regions that retained greater matrix continuity, thereby integrating material modification with through-thickness structural regulation. The gradient laminate showed more segmented crack paths and damage dispersion while suppressing continuous delamination. In the present tests, the gradient CFRP laminate exhibited a Mode I interlaminar fracture toughness of 1.342 kJ·m^-2 (n = 1) and a mean CAI strength of 192 MPa (n = 3), corresponding to descriptive increases of approximately 148% and 113.3%, respectively, relative to the unmodified laminate. These results indicate that the response of the tested laminates depends not only on POSS content but also on its spatial distribution, providing an interfacial-design strategy for high-damage-tolerance CFRP laminates.