Effect of oxygen vacancies on the hot hardness of mullite

The structure of mullite, which has a composition ranging from 3Al(2)O(3). 2SiO(2) to Al2O3. 2SiO(2), contains ordered oxygen vacancies. Sillimanite, Al2O3. SiO2, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al(2)O(3). 2SiO(2)) was measured fro m room temperature up to 1400 degreesC and compared with that of single-cry stal sillimanite (Al2O3. SiO2) up to 1300 degreesC. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and t herefore that the oxygen vacancies in the mullite structure are not the pri mary cause of mullite's resistance to high-temperature deformation.