Effect of TiO2 on the mullite formation and mechanical properties of alumina porcelain

The effect of adding TiO2 on the microstructure and flexural strength of standard alumina porcelain was investigated. A series of alumina porcelain bodies containing increasing amounts of TiO2 were prepared by extruding mixtures of raw materials and TiO2. Porcelain rods were fired under industrial scheduling in a kiln. The overall degree of crystalline and amorphous phase content within the porcelain bodies was quantitatively determined using Rietveld analysis. The results indicated a higher amount of mullite formation in porcelain bodies containing TiO2. Examinations of the products, using FESEM, showed a high density of secondary mullite crystals was present in the earlier feldspar grain areas of the specimens containing TiO2. EDX of secondary mullite crystals revealed that Ti4+ entered into the secondary mullite structure forming a Ti4+-mullite solid solution. Assessment of the mechanical properties of the TiO2-containing bodies indicated that small additions raised the flexural strength of the standard porcelain. The improvement in mechanical properties could be associated with an increase of both specimen density and relative content of types II and III secondary mullites. Both observations were attributed to a decrease in the viscosity of the melted feldspar grains, which in turn favoured the nucleation and growth of secondary mullite crystals and thus increased the final density of porcelain bodies.