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

• • Vickers hardness reaches 155 GPa at ~30 GPa and 1400 °C, exceeding natural cubic diamond by >40%, but this value is anisotropic and obtained on a millimeter-sized bulk; industrial adoption requires isotropic hardness and reproducible scaling to centimeter dimensions. • • Synchrotron XRD shows interlayer spacing of graphite at 30 GPa (and even 50 GPa) remains >2.5 Å, contradicting the <2.5 Å threshold required for direct C–C bond formation between layers; this invalidates the proposed direct post-graphite-to-HD transformation mechanism and necessitates alternative coherent interface models. • • Low synthesis temperatures (e.g., 1400 °C) are mandatory to achieve high HD volume fractions, but they result in poorly sintered large-angle grain boundaries, reducing impact toughness below practical thresholds; high-pressure flash sintering is identified as a potential remedy. • • The reported HD bulk is derived from highly oriented pyrolytic graphite (HOPG) precursor, and the process yields a highly oriented microstructure; industrial viability hinges on replacing expensive HOPG with cost-effective polycrystalline graphite while maintaining >90% HD phase purity and hardness >100 GPa.