Artificial nacre-like layer using layer coating with bioinspired mesolayer insertions

Abstract

The fabrication of coatings using conventional methods [e.g., physical vapor deposition (PVD) and/or pulsed laser deposition (PLD)] usually creates clear interfaces between the edges. It is challenging to increase the toughness by adding soft materials into hard materials to reinforce the structures using architectural design. In this study, we determined that by adding soft materials into hard materials, we can simulate nacre, which results in stronger coatings. However, the compatibility of materials needs to be considered (i.e., the bonding of organic/inorganic components to each other). Titanium dioxide (TiO2), an inorganic material, was chosen as the hard part, and polyimide (PI), an organic material, was chosen as the soft part. When we fused these two materials together to form a layer-by-layer coating, the bonding between them was not good, which degraded the mechanical performance. Nevertheless, we revaluated the differences between artificial and natural coatings. The layers in nacre smoothly change from chitins to proteins or from proteins to chitins owing to the seasonal environmental temperature changes. Inspired by this concept and by simulating nature, we created mesolayers to adhere two different materials and to enhance the coatings that are made entirely of inorganic or organic materials to create new composites using the PVD/PLD method. The mesolayers mixed with two materials at proper ratios were inserted into the organic/inorganic (PI/TiO2) interfaces as bridges to cross-link the materials and dramatically improve the mechanical performance, which is similar to the structure of natural nacre.