Synthesis of Millimeter-Sized Hexagonal Diamond Bulk from Highly Oriented Pyrolytic Graphite at 30 GPa and 1400 °C: A Critical Commentary on Phase Transformation Mechanisms and Sintering Challenges
The recent synthesis of millimeter-sized hexagonal diamond (HD)-dominated bulk from highly oriented pyrolytic graphite (HOPG) at ~30 GPa and 1400 °C by Liu, Yao and co-workers represents a significant advance in superhard materials. The bulk exhibits a highly oriented microstructure, yielding an anisotropic Vickers hardness of up to 155 GPa, exceeding natural cubic diamond by over 40%. However, the proposed direct transformation mechanism from post-graphite to HD remains contentious. First-principles calculations indicate that C–C bond formation between graphite layers requires interlayer spacing below 2.5 Å, yet synchrotron X-ray diffraction shows that at 30 GPa (and even 50 GPa) the interlayer spacing is substantially larger. This discrepancy challenges the direct phase transformation pathway. An alternative mechanism involving coherent gradia interface formation and subsequent interface advance, previously established for graphite-to-diamond conversion, offers a more plausible explanation. Furthermore, achieving high HD volume fractions necessitates low synthesis temperatures, which impede sintering of large-angle grain boundaries between diamond grains, compromising toughness—particularly impact toughness. Future efforts must prioritize advanced sintering techniques, such as high-pressure flash sintering, to meet practical toughness demands. This commentary assesses the breakthrough, its unresolved mechanistic questions, and the critical barriers to industrial deployment.