Abstract
Crustacean exoskeleton, a natural composite consisting of chitin, proteins and minerals, has gradient constituent and microstructure which provide excellent hardness and fracture toughness. The major toughening mechanism is the crack deflection at the interface between the hard exocuticle and tough endocuticle so that the crack cannot propagate through the exoskeleton directly. The twisted plywood (or Bouligand) structure which possesses elastic modulus mismatch and oscillation could further prevent cracks from propagation. A novel hybrid system combining reactive RF sputtering and pulsed laser deposition is designed and utilized to synthesize bio-inspired ceramic/polymer multilayer coatings. In order to mimic the exocuticle and endocuticle in crustacean exoskeleton, multilayer coatings composed of hard ZrO2 outer layers and tough TiO2/polyimide inner layers were synthesized. The thickness ratio of TiO2/polyimide layers is kept 10 to 1 (100 nm/10 nm), and the thickness of ZrO2 was altered from 100 nm to 500 nm to investigate the effect of the elastic modulus gradient on the mechanical properties. Nanoindentation was conducted to evaluate the mechanical performance of multilayer films. The fracture toughness of bio-inspired coatings was further evaluated by the energy-based indentation method. Results show that the multilayer film with specific thickness ratio of ZrO2 has the highest fracture toughness. Toughening mechanisms were elucidated and optimal bio-inspired designs were proposed in this study.