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

The effect of adding TiO2 to standard alumina porcelain on its microstructure and flexural strength 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 manufacturing kiln. The overall degree of crystalline and amorphous phase content within the porcelain bodies was quantitatively determined using a Rietveld analysis. Results indicated a higher amount of mullite formation in porcelain bodies containing TiO2. Examination of the product materials using field emission scanning electron microscopy showed a high density of secondary mullite crystals present in the earlier feldspar grain areas of specimens with TiO2. Energy dispersive X-ray analysis of secondary mullite crystals revealed that Ti4+ enters into the secondary mullite structure forming a Ti4+-mullite solid solution. Assessment of the mechanical properties of the TiO2 containing bodies indicated that small addition raises 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 may be attributed to a decrease in the viscosity of melted feldspar grains, which in turn favours the nucleation and growth of secondary mullite crystals and thus increases the final density of porcelain bodies.