Strategies for simultaneous strengthening and toughening via nanoscopic intracrystalline defects in a biogenic ceramic

Abstract

While many organisms synthesize robust skeletal composites consisting of spatially discrete organic and mineral (ceramic) phases, the intrinsic mechanical properties of the mineral phases are poorly understood. Using the shell of the marine bivalve Atrina rigida as a model system, and through a combination of multiscale structural and mechanical characterization in conjunction with theoretical and computational modeling, we uncover the underlying mechanical roles of a ubiquitous structural motif in biogenic calcite, their nanoscopic intracrystalline defects. These nanoscopic defects not only suppress the soft yielding of pure calcite through the classical precipitation strengthening mechanism, but also enhance energy dissipation through controlled nano- and micro-fracture, where the defects’ size, geometry, orientation, and distribution facilitate and guide crack initialization and propagation. These nano- and micro-scale cracks are further confined by larger scale intercrystalline organic interfaces, enabling further improved damage tolerance.

Biomineralized Materials as Model Systems for Structural Composites: Intracrystalline Structural Features and Their Strengthening and Toughening Mechanisms

“…[ 22 ] Nanoindentation reveals that biogenic calcite has a higher hardness than geological calcite (measured on different crystallographic planes, Figure 2e). [ 23–28 ] In contrast, biogenic aragonite exhibits (mostly) lower hardness compared to geological aragonite (Figure 2f). [ 29–35 ] Such difference might correlate with the grain sizes and the crystalline features.…”

The mesoscale order of nacreous pearls

“…Mesoscale periodic nacre is a highly ductile structural composite that can withstand mechanical impact and exhibits high resilience on the macro- and nanoscale ( 23 – 25 ). Because of its superior toughness, nacreous shells protect the mollusk’s soft body ( 11 , 26 – 30 ) and inspire scientists designing next-generation supercomposites ( 31 – 33 ). Yet, despite a century of scientific fascination with nacre ( 9 , 11 , 24 , 26 , 34 ), its astonishing mesoscale periodicity has not been quantified, leaving key questions about this material unanswered: does nacre have long-range order?…”

Progressive changes in crystallographic textures of biominerals generate functionally graded ceramics

“…These conclusions also imply that not only species with unidirectionally and near-singlecrystalline bioarmor employ crystallographic control to optimize function. 56 The functionally beneficial change of crystallite orientation during growth generates materials with complex and ostensibly unsystematic texture that stand out with excellent and complex mechanical properties. Such elastic gradients are potentially widespread in protective biomaterials, and we expect that other design principles, such as amorphous-to-crystalline or compositional changes, will also confer increased damage resistance.…”

“…[51][52][53] However, only a few species generate such uniform crystalline prisms, such as P. nobilis or A. rigida, with their [c]-axes parallel to the growth axis of the prisms and, thus, perpendicular to the shell surface. [54][55][56] The calcite prisms of other species are crystallographically more complex, with various mapping experiments demonstrating their polycrystallinity. Thus, these prisms are composed of crystallographically iso-oriented subdomains (or crystallites) with distinct misorientation relative to their neighbors.…”