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Hierarchically structured diamond composite with exceptional toughness, Yue, et al., Zhou, Guo, Tian, Nature, 2020

Novelty/impact/significance:

A nanotwinned diamond (nt-diamond) composite contains a majority of cubic (3C) domains and other non-3C diamond polytypes that are coherently embedded in the former and further interwoven into interlocked nanograins; the structure synergistically enhances the toughness (5 times that of synthetic diamond) and hardness. This provides important guidance in toughening superhard materials/engineering ceramics.

Scientific question:

How to simultaneously improve the toughness and hardness of diamond?

Key of how:

Synthesis of nt-diamond below a certain pressure produces a composite architecture consisting of dominant 3C domains and several non-3C diamond polytypes. This makes crack propagation always in a tortuous manner: zigzag path when entering the nanotwins of the 3C polytype and wavy, sinuous fractures with local transformation into 3C diamond when entering the non-3C polytypes region. This dissipates strain energy effectively, thus enhancing toughness.

Major points:

1. Among material toughening mechanisms, nanotwin structure substantially improves the hardness of diamond without sacrificing the fracture toughness, and the transformation between different polytypes shows a potential for diamond toughening.

2. To synthesize nt-diamond, a low pressure below 18 GPa generates composites consisting of nt-diamond (3C type) and M-diamond (non-3C polytypes) domains, which shows fracture toughness as high as 26.6 MPa, 5 times that of synthetic diamond, due to complex crack propagation behaviors.

3. The nt-diamond composite has a main phase of nanotwin structure (3C), stacking faults (SF) and the non-3C diamond polytypes that interface coherently with each other and with 3C diamond (PS: very beautiful TEM images and diffraction patterns).

Multiply nanotwin domains are interwoven, some share common {111} planes while others through different twin boundaries and localized atomic rearrangements.

4. The non-3C diamond polytypes, 2H, 4H, 9R, 12R characterized by HAADF-STEM images and FFT patterns, form coherent interfaces with neighboring 3C domains (stacking sequences of the polytypes are further corroborated by simulated electron diffraction patterns).

The d-spacings are all analyzed and correspond to 3C and non-3C diamond polytypes. These structural observations provide concrete evidence of diamond polytypes that have long been debated.

5. Pressure is essential in controlling the polytype formation; it is proposed that some carbon layers may not gain sufficient driving force to turn into the stable 3C structure at lower pressure in this work. Such pressure-induced transformation heterogeneities also occur in other materials, wurtzite-type BN or cubic BN.

6. This diamond composite, consisting of non-3C diamond polytypes embedded in 3C nanotwins and further interwoven into interlocked nanograins with low-energy boundaries, exhibits high hardness (similar to that of nt-diamond) and high fracture toughness in SENB in SEM (five times that of synthetic diamond and much higher than pure nt-diamond) and indentation toughness.

This high toughness is most likely due to the presence of non-3C polytypes.

7. In-situ fracture in TEM reveals that: in the 3C region, the crack is transiently arrested at twin boundaries and deflected into next twin domain (different twin domains have its {111} cleavage planes oriented differently); this crack arrest and deflection repeat continuously as the crack passes through different nanotwins, leading to a zigzag propagation path.

In the non-3C polytype regions, because these domains share coherent interface with the 3C diamond, each only have one main slip system parallel to the interface, and there are multiple different polytype domains with various stacking sequences, the crack deflects continuously at nanoscale, leading to a wavy, sinuous fracture surface. In addition, a structural transformation from non-3C polytype into stable 3C diamond is revealed as a toughening mechanism.

The larger deflection in bending of the nt-diamond composite than the pure nt-diamond is additional toughening and the partial self-heal of crack tip is one favorable property.

The work is beautiful, and it can be imaged that there were a lot of work behind the statements and the figures.

Here is the link of the fulltext: https://www.nature.com/articles/s41586-020-2361-2#Abs1