Processing and Fracture Mechanics of Multiscale Nacre-like Composites

Abstract

Nacre, aka mother of pearl, is a biological composite with an exceptional combination of stiffness, strength and fracture toughness, despite being 95% calcium carbonate, a mineral not known for any of these properties. This suggests that nacre’s exceptional properties are structural rather than chemical in origin, which has attracted the attention of the composite community as a potential source of valuable design principles for the rational design of new composites. Though primarily presented as a “brick and mortar” arrangement of close-packed microscale mineral platelets interlayered with a thin bio-polymer matrix, the structure of nacre is hierarchical. At the nanoscale, small continuous mineral bridges and surface nano-asperities decorate platelet interfaces, and at the millimeter scale, nacre has a layered structure, where brick and mortar subdivided into 300 µm thick lamella, separated by seasonal growth bands of biopolymer. Although these different levels of structural hierarchy have been well characterized, there is a limited understanding of how they interact to define nacre’s strength, stiffness and fracture toughness. A simplified physical model of nacre would be a powerful tool for exploring structure property relationships at all three levels of hierarchy and better understanding the global design principles in nacre. Towards these goals, a multi-step processing route was established for the production of bulk polymer matrix composites with nacre-like mineral architecture, such that each level of structural hierarchy could be independently controlled. At the microscale, appropriately sized brick and mortar morphology was generated by magnetic alignment of preformed mineral platelets into mineral scaffolds. Partial sintering under uniaxial pressure, produced scaffolds with sufficient mineral density to have consistent platelet-platelet interfaces. By selecting mineral platelets pre-coated with a second phase of lower sintering mineral, temperature dependent selective sintering could be employed to regulate nanoscale mineral bridge and surface asperity morphology at the platelet-platelet interface. Subsequent infiltration of these scaffolds with the desired polymer generated bulk, polymer matrix composites with nacre-like nano- and micro-structure. Systematic variation of the mineral bridge fraction within a series of iso-dense scaffolds sintered at varying temperatures allowed us to demonstrate a linear relationship between mineral bridge fraction and composite strength following an adapted shear-lag model. This model shows that the limit on ultimate achievable composite strength is a simple function of the platelet volume fraction and the intrinsic strength of the platelet, and can thus be manipulated by size effects and residual stress states. However, until that limit is reached, composite stiffness and strength are primarily determined by the interfacial strength. Since the mineral bridges are an order of magnitude stiffer than the polymer matrix, they dominate the interfacial stress transfer interactions between platelets within the range of platelet volume fractions examined here. While mineral bridges are intact, they bear all the applied load, thus the strength and KIC of the composite trend with the strength of the mineral bridge. Building off this new understanding of composite stiffness and strength, attention was turned to how structural hierarchy could be used to implement nacre’s graceful, non-catastrophic failure behavior, while maximizing composite stiffness and strength. To this end, the bulk nacre-like composite was subdivided into multilayer structures with tough thermoplastic interlayers. The stiffness and strength of these composite laminates are set by the stiffness and strength of the bulk nacre-like layers, according to their rule of mixtures compositions, while the composite failure behavior is defined by the energy absorption capacity of the thermoplastic interlayers as they deform under applied load. This strategy separates the structural features responsible for stiffness and strength onto a different level of structural hierarchy from the structures responsible for fracture toughness and graceful failure behavior. As a result, the first nacre-like composite that combines mineral-like stiffness and strength with plastic fracture toughness has been developed.