. The resultant pellets were finally annealed in air at 950°C for 20 h to remove the effect of carbon diffusion and reduction during SPS process.24 For conventional sintering and two-step sintering,3,25 the dried mixtures were pressed to form disks using 8 wt% polyvinyl alcohol as a binder, and the disks sintered in air by the conventional sintering and two-step sintering, respectively