Reaction sintering process of diamond converted from graphitized pitch coke (GPC) or natural graphite (NG) was examined using the unary (Ni) and binary (Ni-Ti or Ni-Zr) solvent-catalyst systems under high pressure (6.5-7 GPa) and temperature conditions (1600°C-1700°C) for a treatment time of 10 min. Particle joining was extending regions among the diamond grains that were transformed from GPC, while growth of twins was observed in the diamond grains transformed from NG. The diamond sintered compact having more than 90% conversion ratio from GPC, was obtained in the presence of 50-70 wt% (20-37 vol%) solvent-catalyst. The grain size of diamond decreased from 20-40 μm in the unary (Ni) system to 10-20 μm in the binary (Ni-Ti or Ni-Zr) system. The Vickers microhardness of diamond phase of these sintered compacts exceeded 8000, where a typical neck growth between diamond grains was observed. The addition of 30 wt% diamond seed crystals (diameter: 0.5-3 μm) to the starting powder resulted in the further decrease in the transformed diamond grain size to 2-3 μm.
