Impact of surface fluorination on phase stability of nanodiamond particles and inter-granular bonding in polycrystalline diamond under HPHT conditions

Abstract Polycrystalline diamond (PCD) compacts, made by conventional high pressure-high temperature (HPHT) technology, contain Co catalyst in inter-granular spaces causing degradation of PCD mechanical strength and thermal stability during rock cutting due to thermal expansion and graphitization. To solve this problem a catalyst-free synthesis of PCD based on HPHT sintering of fluorinated nanodiamond (FND) and submicron size aluminum mixtures was explored in the present work. The feasibility of using FND has been established by evaluation of phase stability of FNDs of 5, 10, and 90 nm sizes under industrially favorable P-T conditions (7.5–8 GPa, 1500–1700 °C) for PCD synthesis. It was found that surface fluorine slows down the graphitization of nanodiamond particles. Enlargement of diamonds from 5 and 10 nm up to a 10 μm has been observed for FND/Al systems. This phenomenon can be associated with the Wurtz-type reaction under the HPHT conditions leading to formation of inter-granular carbon‑carbon bonds between the nanoparticles and formation of the fluid Al-C-F phases in the samples volume, accelerating the mass transport and re-crystallization of nanodiamond. PCD samples sintered from mixtures of fluorinated micro-diamonds, nano-diamonds FND-90 and 3 wt% aluminum exhibited enhanced up to 1200 °C thermal stability and as high wear resistance as leached commercial PCD.