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High Performance Abrasion-Resistant Materials: Lessons from Nature

Abstract

Chitons are marine mollusks found worldwide in the intertidal or subtidal zones of cold water as well as in tropical waters. These organisms have evolved an amazing feeding structure called a radula. The radula is a ribbon-like structure that consists of abrasion resistant teeth anchored to a flexible stylus that the organism uses to abrade rocky substrates to reach endolithic and epilithic algae. In this work, we investigated the radula in Cryptochiton stelleri, the largest of the chitons. Using various microscopic and spectroscopic techniques, an ultrastructural analysis and investigation of mechanical properties of fully mineralized radular teeth from C. stelleri, as well as the mineralization process of these teeth, has been performed to gain insights into structure-function relationships of the abrasion resistant, impact tolerant, and anti-fatigue composites synthesized by nature. Shape, wear patterns, and feeding positions of the teeth have been analyzed to provide teeth working conditions that can be used in finite element analysis modeling of the tooth, which helps to provide a foundation to produce abrasion-resistant, impact-tolerant and anti-fatigue materials.

An overall analysis of mature radular teeth using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman Spectroscopy revealed a magnetite containing hard shell with an iron phosphate containing soft core. α-chitin was also identified in mature teeth with powder XRD. Energy-dispersive X-ray spectroscopy (EDS) analysis uncovered the higher carbon concentration at the core region than at the shell. Hardness and modulus gradients from the leading edge to the trailing edge of the teeth, which contributes to a self-sharpening mechanism, were revealed by nanoindentation. The mechanical measurements revealed the teeth had the highest hardness (12.5 GPa) and modulus (125 GPa) of any biomineral reported to date.

Phase transformation and structural development during the biomineralization process of the radular teeth were analyzed with EDS, synchrotron XRD, Micro- X-ray Fluorescence (µXRF), SEM and TEM. The phase transformation from ferrihydrite to magnetite, which occurred at the tip of the leading edge, along with aggregation of mineral particles and shrinkage of organic fibers were discovered.

Regional micro- and nanostructures as well as the mechanical properties of the mature teeth were characterized via SEM analysis of the fracture surfaces, TEM and nanoindentation. Nanorods structures were found at the trailing edges and the surfaces of the leading edges of teeth. The middle of the leading edge of the tooth is made of close-packed particles and exhibited the highest hardness and modulus. The rods at the trailing edge were orientated along the surface curvature of the trailing edge, while the rods at the surface of the leading edge were rotated 90°. The diameters of the rods at the trailing edge were larger than those at the leading edge. Tooth performance was simulated with FEM analysis on a radular tooth model obtained from confocal microscopy imaging. Tensile stresses were concentrated at the leading edge, while the compressive stresses were concentrated at the trailing edge. Both of the stress directions are guided along the surface curvature, following the major orientation of the rods. The combination of regional ultrastructural features of these teeth, consisting of the highest hardness and modulus mineral known in Nature, enables the radular teeth to be optimized abraders during the feeding process.

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