Microstructure characterization and phase transformation kinetic study of ball-milled m-ZrO2–30 mol% a-TiO2 mixture by Rietveld method

High-energy ball milling of monoclinic ZrO2–30 mol% anatase TiO2 mixture at different durations results in the formation of m-ZrO2–a-TiO2 solid solution from which the nucleation of nanocrystalline cubic (c) ZrO2 polymorphic phase sets in. Post-annealing of 12 h ball-milled sample at different elevated temperatures for 1 h results in almost complete formation of c-ZrO2 phase. Microstructure of the unmilled, all the ball milled and annealed samples has been characterized by Rietveldʹs X-ray powder structure refinement method. Particle size, rms lattice strain, change in lattice parameters and phase content of individual phases have been estimated from Rietveld analysis, and are utilized to interpret the results. In course of milling, (1 1 1) of cubic lattice became parallel to (View the MathML source) plane of monoclinic lattice due to the orientation effect and cubic phase may have been formed on the (0 0 1) of the m-ZrO2–a-TiO2 solid solution lattice. A comparative study of microstructure and phase transformation kinetics of ZrO2–10, 20 and 30 mol% a-TiO2 ball-milled and post-annealed samples reveals that rate of phase transformation m→c-ZrO2 increases with increasing a-TiO2 concentration and ∼30 mol% of nanocrystalline c-ZrO2 phase can be obtained within 4 h of milling time in the presence of 30 mol% of a-TiO2. The post-annealing treatment at 773, 873 and 973 K for 1 h duration each reveals that rate of c-ZrO2 formation with increasing temperature is retarded with increasing a-TiO2 concentration but the amount of c-ZrO2 becomes almost equal (∼95 mol%) at 973 K. It suggests that almost fully stabilized nanocrystalline c-ZrO2 can be formed by adding a tetravalent solute to m-ZrO2.