Creep of Mullite Ceramics

Abstract

Creep in mullite ceramics containing 71, 74 and 78wt% Al₂O₃ made from coprecipitated powder has been studied between 1350° to 1500°C. Microstructure of 71wt% Al₂O₃ mullite was characterized by rod-like and equiaxed grains (average grain size: 2.13μm), that of 74wt% Al₂O₃ mullite, by equiaxed grains (grain size: 5.44μm), and that of 78wt% Al₂O₃ mullite, by equiaxed mullite grains (grain size: 2.11μm) and Al₂O₃ particles of 1.31μm in grain boundaries. Creep rate in 74wt% Al₂O₃ mullite was the lowest and that of 71wt% Al₂O₃ was the highest. That of 78wt% Al₂O₃ mullite located in between. Activation energies for creep were 138, 168 and 119kcal/mol for 71, 74 and 78wt% Al₂O₃ mullite, respectively. Vertical offsets from grain sliding out of the specimen surface were observed after creep test. The number of grains indicating the vertical offsets increased and cavities evolved with increasing creep time or applied stress. A slight grain growth was observed for 71 and 78wt% Al₂O₃ mullites after creep tests. On the other hand, grain growth was not recognized for 74wt% Al₂O₃ mullite. It was proposed that the creep mechanism in mullite ceramics is grain boundary sliding accompanied by diffusion.