Fabrication of Oxide Ceramics with High Fracture Strength and High Thermal Shock Resistance

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In order to improve mechanical properties and thermal shock resistance of tetragonal zirconia polycrystals with high fracture strength and toughness, thermal stress fracture behavior and mechanical properties of yttria-doped tetragonal zirconia polycrystals (Y-TZP) and Y-YZP based ceramics were investigated. The thermal stress fracture of ceramic materials in the water-quenching test was initiated by the thermal stress due to the convective heat transfer accompanied by boiling of water. The thermal shock resistance of ceramic materials increased with increasing the fracture strength, thermal conductivity and thermal transfer coefficient and with decreasing Young's modulus, Poison's ratio and thermal expansion coefficient of the material. The thermal shock resistance of Y-TZP based ceramics was anormalously lower than the predicted value, because the toughening mechanism by the stress-induced tetragonal to monoclinic phase transformation did not function well against the thermal stress … More in Y-TZP ceramics. The increment of the grain size of Y-TZP ceramics increased the contribution of the transformation toughening against not only the mechanical-stress fracture but also the thermal-stress fracture, which consequently improved both fracture toughness and thermal shock resistance. Therefore, the thermal shock resistance of Y-TZP was correlated with the fracture toughness rather than fracture strength. Dispersion of A1_2O_3 particles into Y-TZP was useful for improving both the fracture strength and thermal shock resistance. On the other hand, dispersion of mullite particles and SiC whiskers into Y-TZP resulted in decreasing the relative density and fracture strength but in increasing the thermal shock resistance, because of the decrease of the thermal expansion coefficient and of the increase of the thermal conductivity. Hot isostatic pressing of the presintered body was useful to increase the relative density without the grain growth and to improve the fracture strength and thermal shock resistance. Less