Biominerals with Texture Gradients are Functionally Graded Bioceramics Toughened by Stress Delocalization

TL;DR

This study reveals that biominerals with texture gradients, like those in Pinctada margaritifera, are a new class of functionally graded bioceramics that achieve toughness through stress delocalization, offering insights for bioinspired material design.

Contribution

It uncovers the existence of texture-gradient biominerals as a novel class of functionally graded materials with enhanced mechanical properties.

Findings

Biominerals with crystal texture gradients are functionally graded materials.

Texture gradients improve toughness by stress delocalization.

These principles can inspire new bioinspired materials.

Abstract

Biomineralizing organisms are widely noted and extensively studied due to their ability to generate structures exhibiting exceptional crystallographic control. Primarily, it is the organisms, such as sea-urchins or bivalves, that generate nearly single-crystalline biocrystals that have attracted attention. In contrast, biomineralizing organisms with seemingly disordered polycrystalline bio-armor have been left relatively unstudied. However, the crystalline ordering in the black-lipped pearl oyster, Pinctada margaritifera, reveals that biominerals with varying crystal textures are an unrecognized class of functionally graded materials. Changing crystal textures inevitably cause a variation in Young modulus due to the orientation-dependent mechanical properties of crystals. The case of Pinctada margaritifera demonstrates that bioceramics with such crystallographical gradients are toughened by stress delocalization and reduced stress intensity factors, outperforming non-graded counterparts. These findings suggest that a multitude of biominerals, which are perceived as poorly ordered because of their polycrystallinity and changing crystal textures, may be considered as graded materials with hitherto unidentified emergent mechanical properties. The underlying design principle is remarkably simple and applicable to a wide range of crystalline material classes and may thus serve as a blueprint for future bioinspired functional materials.