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Bioinspired hybrid materials from spray-formed ceramic templates, Dwivedi, Flynn, Resnick, Sampath, Gouldstone, Advanced Materials, 2015


Different from most research that explore exquisite fabrications, an industrial thermal spray-based technique is developed for making nacreous brick-and-mortar alumina composites, which allows for controlled architecture across length scales, diverse constituent materials, and easy scaling-up.

The brick-and-mortar alumina infiltrated with polymer (4-5 vol%) show higher flexural strength than nacre, and the flexural strength and fracture toughness are eight and six times than the pure staggered/lamellar alumina.


Scientific question:

How to address the essential challenges of architectural control, scalability/mass-productivity and material flexibility in making nacre-mimetic composites?


Key of how:

Using the thermal spray technique through optimizing processing parameters, a brick-and-mortar structure consisting of staggered microscale alumina lamellae is fabricated; then infiltrating 4-5 vol% polymer produces a nacre-like composite, in which similar strengthening and toughening mechanisms (e.g., crack deflection, platelet sliding, pullout/fracture, polymer bridging) take effect.


Major points:

1. Nacre is one typical natural strong and tough material despite composing of 95+% aragonite, due to its brick and mortar architecture (staggered mineral platelets within an organic matrix).

2. A variety of laboratory techniques have been developed to achieve the interlayered architecture with diverse materials, e.g., layer-by-layer assembly, synthetic mineralization, and freeze casting. Especially, the freeze-cast Al2O3 with PMMA and further an all ceramic-based composite show superior strength and toughness values.

3. In making nacre-like structured materials, the architecture of constituents and the scalability/mass-productivity with material flexibility are still important challenges.

An industrial based thermal spray (ST) technique is explored here. It inherently produces thick free-standing coatings that show a lamellar structure (each lamella/splat resembles one platelet), resulted from the high-temperature-melted micrometer-sized droplets impinging a substrate, spreading and solidifying to form the ‘staggered bricks’, the lamellae, with wavy/rough interfaces and interlamellar gaps.

These ceramic lamellar templates alone show significant inelasticity and hysteresis in flexure (interfacial sliding), which differs clearly from the bulk ceramic.

4. When infiltrated with polymer, the ceramic composites display a brick-and-mortar structure and show dramatically enhanced flexural strength and fracture toughness, which are on par with/superior to existing successful nacre-inspired composites and corroborate the structural effects in improving mechanical properties.

5. By varying feedstock in flame spray deposition and subsequent polymer infiltration, disordered alumina template, lamellar alumina template, disordered alumina-epoxy composite, and brick-and-mortar alumina-epoxy composite are fabricated.

From the disordered to the lamellar templates, flexural strength and fracture toughness increase; the polymer infiltration further substantially enhances both values for the brick-and-mortar structures, ~800% and 600% improvements from alumina template to alumina-epoxy composite, despite the little content of epoxy (4-5 vol%, similar to the organic volume fraction in nacre).

6. The strengthening and toughening mechanisms are similar to those in nacre, e.g., crack deflection, interlayer/platelet sliding through rough wavy interfaces, polymer bridging, lamellar pullout. Much of the inelastic accommodation appears to be local and the stress-strain curves do not indicate the process zone toughening for a rising R-curve.

With referencing to a prior work reporting the fracture behavior of thermal sprayed ceramics, it is suggested that tailoring the interfacial properties and the splat/platelet geometry can further improve the mechanical performance. And by introducing multi-level structure via changing processing parameters, the fracture toughness can also be enhanced.

7. The fracture toughness and flexural strength of the thermal sprayed brick-and-mortar alumina-epoxy composites are both among the highest values in current nacre-related ceramic-polymer composites.

This method, already generating composites with excellent properties, is preliminarily explored here; it allows for wide selection of materials (i.e., metals to further tune the properties), easy to scale-up (industrial technique), and is versatile in setup to construct diverse architectures/structures.

By combining different thermal spray strategies, a bilayer structure consisting of a fully dense stiff topcoat on an inner brick-and-mortar layer is fabricated, resembling the widely seen hard shell-over-tough core structures in nature.

The bilayer structure shows a stiff response followed by a graceful failure with large deformation for energy dissipation, a favorable mechanical behavior desiring further exploration.  It is promising and exciting.

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