Effect of micro-diamond and nano-polycrystalline diamond interfacial microstructure on OLC phase transition

•Micro-diamond (MD) disrupts the symmetric equilibrium of the complete structure onion-like carbon (OLC).•MD has changed the initial path of the diffusionless phase transition from OLC to nano-polycrystalline diamond (nPCD).•MD and nPCD have formed interfaces of twin boundaries, cubic diamonds, stacking faults, and amorphous grain boundaries. The interfacial microstructure of micro-diamond and nano-polycrystalline diamond (nPCD), as well as their effect on the phase transition of complete structure onion-like carbon (OLC), were investigated using molecular dynamics simulations and experimental methods under high pressure and temperature (HPHT). Introducing micro-diamond disrupted the symmetric equilibrium structure of complete structure OLC, reduced the diamond nucleation process, and formed a coherent interface ((111)CD // (002)Gr), where cubic diamond (111) crystal plane was parallel to graphite (002) crystal plane. This transformed the initial diffusionless phase transition path of complete structure OLC into a low potential path for phase transition along the graphite crystal direction [002] by (111)CD // (002)Gr, thereby reducing the phase transition conditions. Additionally, due to different stacking orders or breakages in the graphite layers of the complete structure OLC, micro-diamond, and nPCD formed four interfacial microstructures containing twin boundaries (TB), cubic diamond, stacking fault (SF), and amorphous grain boundary (aB). Furthermore, under HPHT effects, numerous TBs and SFs were generated inside micro-diamonds as well as at their edges, resulting in a Vickers hardness of 167 GPa for polycrystalline diamond (PCD) composite. This simple yet effective method reduces synthesis conditions for OLC precursors while producing high-performance PCD composites with promising applications.