Creep and Creep Fracture of Oxide Ceramics

The effects of microstructures(grain size，second phase，etc)on high temperature deformation such as creep and creep fracture were studied for some oxide ceramics；magnesias with and without an intergranular glass phase，a fine-grained，yttria stabilized tetragonal zircoenia exhibiting superplasticity，a beta-spodumene glass ceramic and fine-grained，single phase mullite(3Al_2O_3·2SiO2_2_2)。Glass phases in the magnesia and the glass ceramic enhanced the creep strain rates by pressure solution.However，they were the origins of cavitation，resulting in microcracking at grain boundaries。Modern ceramics often have very fine microstructures without apparent glassy phases，thus show sometimes high ductility(Superplasticity In Some Cases)at high temperatures，leading the application to plastic forming。On the other hand，these ceramics show limited resistance to deformation in high-temperature structural application。The mullite was shown to have high creep resistance in spite of its fine grain size(1.5 mum)and to have high strength in creep fracture.The increase in grain size of the mullite led to cavitation at grain boundaries and thus lowered the creep fracture stress，as observed in Y-TZP.The criterion for initiation of cavitation was difficult to determine，which is the next step of this study.

The effects of microstructures (grain size, second phase, etc) on high temperature deformation such as creep and creep fracture were studied for some oxide ceramics; magnesias with and without an intergranular glass phase, a fine-grained, yttria stabilized tetragonal zircoenia exhibiting superplasticity, a beta-spodumene glass ceramic and fine-grained, single phase mullite (3Al_2O_3・2SiO_2). Glass phases in the magnesia and the glass ceramic enhanced the creep strain rates by pressure solution. However, they were the origins of cavitation, resulting in microcracking at grain boundaries. Modern ceramics often have very fine microstructures without apparent glassy phases, thus show sometimes high ductility (superplasticity in some cases) at high temperatures, leading the application to plastic forming. On the other hand, these ceramics show limited resistance to deformation in high-temperature structural application. The mullite was shown to have high creep resistance in spite of its fine grain size (1.5 mum) and to have high strength in creep fracture. The increase in grain size of the mullite led to cavitation at grain boundaries and thus lowered the creep fracture stress, as observed in Y-TZP. The criterion for initiation of cavitation was difficult to determine, which is the next step of this study.