Reaction mechanisms of synthesis and decomposition of fluorapatite–zirconia composite nanopowders

The reaction mechanisms of formation and decomposition of fluorapatite−zirconia composite nanopowders were investigated after the mechanochemical process and subsequent thermal treatment. Experimental results indicated that formation of fluorapatite−zirconia composite nanopowders proceeded in several steps. In the first stage, phosphoric acid formed immediately upon addition of phosphorous pentoxide to the reaction mixture. Afterwards, anhydrous dicalcium phosphate was generated as a result of reaction between reagents with phosphoric acid. The synthesis progressed by the formation of the stoichiometrically deficient hydroxyfluorapatite−zirconia composite at milling times between 5 and 15 min. Ultimately, the fluorapatite−zirconia composite nanopowder was obtained after 300 min of milling. Results revealed that the annealing process led to a decomposition of fluorapatite to tricalcium phosphate and calcium fluoride, and to the transformation of monoclinic zirconia to the tetragonal form. Field emission scanning electron microscope observations showed that the milled sample was composed of fine particles with a mean particle size of about 45 nm after 300 min of milling. Besides, the mean particle size increased progressively due to crystal growth in the temperature range above 900 °C. According to the gained data, reaction mechanism steps were proposed to clarify the reactions occurring during the above-mentioned solid state process.