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Hierarchical toughening of bioinspired nacre-like hybrid carbon composite, Jiao, Zhang, Liu, Liu, Zhang, Tang, Liu, Zhang, Carbon, 2021


Different from existing nacre-like structures of mineral and organic compositions, a hybrid composite of simple, pure carbon mimicking the brick-and-mortar architecture (graphite flakes in amorphous carbon) is created and exhibits exceptional damage tolerance with a rising R-curve behavior (superior to nuclear-grade graphite), due to effective extrinsic toughening mechanisms at nano- to microscales. This shows a promising strategy for toughening the otherwise inherently brittle carbon-based composites that are important structural materials in aerospace, nuclear, transportation applications.  

Scientific question:

Can the bioinspired nacre-like design be used to toughen the all-carbon based composites?

Key of how:

By freeze casting of graphite flakes with organic components and subsequent carbonization (turning the organic into amorphous carbon), a brick-and-mortar carbon composite is obtained with hierarchical lamellar structure (the micron flakes inherently consist of nanoscale lamellae and inherit to the composite).

The potent extrinsic toughening mechanisms including crack deflection, formation and pull-out of ligament bridges are closely related with the structure and lead to the exceptional damage tolerance.

Major points:

1. Graphite-based composites have remarkable thermal, electrical and mechanical properties that are essential to high-end applications such as aerospace vehicles and nuclear reactors. But they suffer from inherent brittleness due to the bonding nature (strong covalent bonds within layer and weak van der Waals between layers).

2. Nacre is a representative in displaying exceptional toughness via extrinsic toughening mechanisms arising from its staggered alignment of submicron calcium carbonate platelets (95wt%) with little organic in between.

3. Replicating the nacre-like brick-and-mortar structure has seen success in toughening composites mostly composed of ceramics and organics (and even all ceramic composites); while whether such a strategy can be effective for composites with single constituent/element has not been realized.

Also, the achieving structural hierarchy is not easy, with current few works using 3D printing or complicated alternative stacking.

This work deals with the above aspects, by implementing the nacre-like structure into an all-carbon composite with significantly enhanced fracture toughness.

4. The fabrication is seemingly straightforward, preparing the slurry (graphite flakes and organic components), bidirectional freezing casting, compressing, sintering, carbonizing (turn the organics into amorphous carbon), then characterization, testing, evaluation. While parameters in freeze casting are important for the lamellae size, alignment, and parameters in compressing and carbonization for the final graphite-amorphous carbon structure.

5. The freeze-dried, ice-templated scaffolds show a long-range aligned lamellae of graphite flakes. Carbonized composites show numerous nanoscale platelets (from the micron graphite flakes) stacked and aligned in plane, resembling the nacre structure despite different chemical composition and length scale.

Three levels of hierarchy exist: the coarsest scale, crystalline graphite flakes/domains (several µm thick) in staggered arrangements, in between is an amorphous interface; within each domain are parallel layers with ~100 nm thickness, and further each layer split into lamellae having a thickness of ~10 nm. This multi-layered structure is originated from that of graphite flakes.

PS: what are the interfaces between the layers and between the lamellae, e.g., also amorphous carbon?

The density is 1.89 g/cm3, similar to that of nuclear graphite.

6. The nacre-like hybrid carbon composites show a stress-drop (corresponding to the onset of global cracking) and then increasing load-bearing ability in flexural stress strain curves, with a strength of 41 MPa, comparable to most nuclear graphite materials.

The composites show a rising R-curve behavior, with a critical crack initiating J-integral of 74 J/m2 and a J-based fracture toughness of 140 J/m2. These are higher than the graphite flake/organic composite before the matrix carbonization, which is ascribed to the improved strength and stiffness of the amorphous carbon matrix compared to the organic matrix. Also, the fracture toughness and strength are comparable or even higher than those of most graphite materials.

7. The composite’s nacre-like structure activates a series of extrinsic toughening mechanisms to enhance the damage tolerance. The zig-zag cracking profiles indicates toughening mechanisms ranging multi-levels, e.g., ligament bridges, pull-out, and crack deflection at microscale, and crack bridging by lamellae at nanoscale.

This work demonstrates the successful implementation of nacre-like structure into carbon-based composites with favorable mechanical efficiency, considering that the generated carbon composites show superior mechanical properties to higher quality graphite materials, in spite of using low-cost with limited purity graphite flakes.

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