Synthesis and characterization of nacre-inspired zirconia/polyimide multilayer coatings by a hybrid sputtering and pulsed laser deposition technique

Abalone nacre is a natural ceramic-based composite consisting of 95 wt.% stacked CaCO3 tiles and 5 wt.% organic layers organized into a unique multilayer structure, which leads to exceptional fracture toughness. Multi-scale toughening mechanisms such as crack deflection at the organic/inorganic interfaces, viscoelastic organic glue, nano-asperities and interconnected mineral bridges between tiles, collaborate synergistically to prevent deformation and failure. Inspired from abalone nacre, multilayer coatings of zirconia and polyimide layers were synthesized by a hybrid PVD system combining sputtering and pulsed laser deposition. By introducing thin polyimide interlayers between zirconia layers, the fracture toughness of multilayer coatings (5.2 MPa.m(1/2)) was significantly enhanced, approaching six times higher than that of zirconia monolayer (1.0 MPa.m(1/2)). The thickness ratio of zirconia and polyimide was kept 10:1 while thickness and number of interfaces were altered to investigate the effect of organic/inorganic interfaces on the mechanical properties of the coatings. Results showed that multilayer structure could enhance the fracture toughness of coatings. Fracture toughness significantly increased with increasing number of interfaces yet the hardness slightly decreased. SEM observation verified that the major toughening mechanism of bio-inspired multilayer coatings was crack deflection at organic/inorganic interfaces, which prevented crack from direct propagation. With certain critical interfacial roughness, fracture toughness of multilayer can be further improved, similar to the function of nano-asperities in abalone nacre. Bio-inspired organic/inorganic multilayers could improve the toughness of intrinsically brittle ceramic or glass coatings and extend their applications in protection, wear and corrosion resistance, optical and biomedical fields. (C) 2015 Elsevier B.V. All rights reserved.