A natural impact-resistant bicontinuous composite nanoparticle coating, Huang, et al., Kisailus, Nature Materials, 2020
Novelty/impact/significance:
The rarely known impact surface of the widely reported shrimp dactyl club is a thin, nanoparticle-based coating, possesses both high stiffness and damping properties under high strain rates for exceptional impact resistance, and provides new insights in designing advanced engineered armor materials.
Scientific question:
In terms of high strain rate behavior how does the shrimp dactyl club structure resist impacts?
Key of how:
The coating consists of densely packed (88%) bicontinuous nanoparticles (hydroxyapatite, HAP) integrated within an organic matrix (chitin and proteins), and the nanoparticles are further composed of highly aligned HAP grains. This particle system is an interpenetrating, bicontinuous network in 3D of organic and inorganic phases (organic phase exist within the grain lattice and/or between grains, and between particles).
The structure leads to high stiffness and damping to localize damage and avoid catastrophic failure from high-speed collisions, through mechanisms including particles fracture into grains/rotation and translation, randomization of the grain orientation, impact induced crystal imperfections (dislocation and amorphoization), and additional toughening through the deformable organic proteins.
Major points:
1. Through high strain-rate impacts, the mantis shrimp dactyl club can fracture the widely known, strong and tough biological composite, the mollusc shells.
The dactyl club is reported to have a layered structure: (I) impact region of mineralized fibers in a herringbone architecture, then (II) periodic region of helicoidal arranged chitin fibers within a mineral matrix, and (III) circumferential, striated region of highly aligned chitin fibers wraping around the club. The impact and periodic regions deflect and twist cracks, while the striated region constrains the crack opening and keeps the club in compression during impact, thus increasing toughness.
But the essential and important effects of high-strain-rate impacts are not known.
2. Intact (newly formed) dactyl club has an outermost, 70 µm-thick coating (surface), which gets worn after heavily used. It consists of ~65nm crystalline HAP nanoparticles (~88% volume fraction) embedded in an organic matrix (chitin proteins); each of the particles is further composed of ~16 nm highly aligned HAP grains (with low angle grain boundaries in between). These grains colocate with an organic phase, a hydrated chitin and protein mixture (17%, verified by TEM, AFM, HRTEM, FTIR, TGA and DSC), that is, this particle system (the nanoparticles and grains) consist of an interpenetrating, bicontinuous network in 3D of organic and inorganic phases (organic phase present within grain lattice and/or between grains, and between particles).
Very beautiful TEM images with clear notations and FFT patterns.
3. Microimpact tests (~104 s-1) with different indentors reveal that the dactyl impact surface with a bicontinuous nanocomposite structure shows significantly localized damage and limited crack propagation, compared with the impact region and other typical biological and engineering materials (particle pile-up v.s. crack propagation, one half penetration depth, twice energy absorption density, smallest damage area, etc.).
While the impact surface exhibits more cracks initiation and propagation under static indentation.
It is likely that chitin and proteins stiffen under high strain rates, leading to the localized failure of the particles and limited cracking between particles.
The impact surface also shows a superior damping property than most engineered metals and composites with similar stiffness (e.g., larger loss tangent than aluminum and fused silica) to deal with high accelerations and impacts.
4. The energy dissipation mechanisms are nanoparticle fracturing into grains, randomization of the grain orientation, and impact induced crystal imperfections (dislocation and amorphoization), observed by SEM, HRTEM.
In situ TEM compression reveals large deformation and high recoverability, attributed to the bicontinuous nature of the hydrated organic networks (plastic deformation, fibril bridging). The energy dissipation density is an order of magnitude higher than the bivalve shell.
5. Implementing the bicontinuous design into the nanoparticle structure increases both stiffness and strength compared with the structures with non-integrated, which leads to high energy absorption demonstrated in other materials for engineered and natural composites recently.
6. Molecular dynamics simulations at the nanoscale and atomic scale reveal that the stiffness and strength of the nanoparticles both increase with increasing strain rate, the deformation is more uniform at higher strain rates, and the particle fracture may be the main energy dissipation mechanism.
Very beautiful structures, and the beautiful we see/feel shall imply useful.
Here is the link of the fulltext: https://www.nature.com/articles/s41563-020-0768-7