Effect of glass additives on the strength and toughness of polycrystalline alumina

The effect of stress at grain boundaries on the mechanical properties of alumina ceramics was investigated. Residual stresses at grain-boundaries resulted from a mismatch in thermal expansion coefficient (TEC) between the alumina matrix and the glass-phase segregated at grain-boundaries. The BaO–Al2O3–SiO2 (BAS) system and the Li2O–Al2O3–SiO2 (LAS) system glasses were chosen to have a higher and a lower TEC than that of alumina, respectively, resulting in microscopic tensile and compressive stresses at grain-boundaries for Al2O3/BAS and Al2O3/LAS composites, respectively. The experimental results showed that the Al2O3/BAS composite fractured intergranularly with a fracture toughness higher than that of monolithic alumina. On the other hand, the Al2O3/LAS composite experienced transgranular fracture and high bending strength despite its low toughness. Both composites could be sintered to full density at 1500°C for 2 h due to the presence of a liquid phase. It was concluded that strengthening and toughening of alumina ceramics could be tailored by designing their grain-boundary microstresses.