Hot-pressed AlN body without additive and sintered AlN body containing 10wt% Y2O3 were joined by hot-pressing under a pressure of 200kg/cm2 at 1800°C for 10min. The joined samples with the step-like concentration gradient of grain boundary phases were annealed at 1900°C for 0, 0.5, 1 and 3hrs in N2 using carbon container. The initial migration of grain boundary phases from sintered to hot-pressed body was caused by their concentration gradient. Grain boundary phases transferred to the surface were decomposed and nitrided. After the concentration gradient disappeared, the surface reaction caused the migration of grain boundary phases. The grain boundary phases reacted with oxygen in the hot-pressed body, resulting in the depletion of oxygen in the hot-pressed body.
