Phase transformation kinetic study and microstructure characterization of ball-milled m-ZrO₂-10 mol% a-TiO₂ by Rietveld method

High-energy ball milling of a monoclinic ZrO₂-10 mol% anatase TiO₂ mixture results in the formation of a nanocrystalline cubic ZrO₂ polymorphic phase with equimolar fraction of the starting materials. The cubic phase is presumed to have formed from the m-ZrO₂ solid solution based on the (001) plane of the m-ZrO₂ phase. In the course of milling, the most dense (111) plane of the cubic lattice became parallel to the most dense (11) plane of the monoclinic lattice due to an orientation effect. Annealing of a 12 h milled sample at 773, 873 and 973 K for 1 h results in almost complete transformation of the m-ZrO₂ to the c-ZrO₂ phase. At 1273 K annealing temperature (1 h), the nanocrystalline sample decomposed into individual starting phases. This suggests that the cubic phase is a metastable one and its stability depends on particle size as well as the working temperature. Formation of the cubic phase at such a low temperature using anatase TiO₂ as a phase stabilizer has not been reported previously. The microstructures of the unmilled, all the ball-milled and the annealed samples have been characterized by employing Rietveld's X-ray powder structure refinement methodology. The particle size, root mean square (r.m.s.) lattice strain, lattice parameters, molar fraction, etc., of individual phases have been estimated from Rietveld analysis and are utilized to interpret the results.