Effect of CaO on the Formation of PSZ and γ-Zirconia Nanoparticles through Ball Milling

The effect of CaO on the formation of β- and γ-zirconia nanoparticles from α-zirconia was investigated and their stability evaluated via mechanical activation. α-ZrO2+8.5wt%CaO powder was milled for 2-150 hours with ball-to-powder weight ratios (BPWR) of 40:1 and 60:1. Structural evaluations were conducted using X-Ray diffraction and scanning electron microscope (SEM). Thermal analysis was carried out using Differential Scanning Calorimetry (DSC). Results showed that, γ-zirconia (with a cubic structure) was obtained by ball milling of α-ZrO2+8.5wt%CaO at room temperature. It was found that α to γ phase transformation began after two hours of milling and completed after 48 hours with a BPWR of 40:1. Moreover, the Williamson-Hall equation indicated that the particle size of α-ZrO2+8.5wt%CaO was reduced to 50 nanometers and reached its equilibrium point after 24 hours of milling and was unchanged after that. In addition, DSC analysis showed that β-zirconia and γ-zirconia were unstable in pure ZrO2 powder, while CaO addition led to the stabilization of ZrO2 as β-zirconia and γ-zirconia after ball milling.