Preparation and microstructure characterization of ball-milled ZrO₂ powder by the Rietveld method: monoclinic to cubic phase transformation without any additive

The phase transformation kinetics of high-energy ball-milled monoclinic ZrO₂ have been studied in detail by Rietveld powder structure refinement analysis. In the present study, no stabilizing compound was required to obtain the cubic phase. The fine-grain powder was milled in a planetary ball mill for up to several hours at different BPMRs (ball to powder mass ratios): 10:1, 20:1, 35:1 and 40:1. During the process of ball milling, the monoclinic phase is gradually transformed to the cubic phase. The relative phase abundances of the respective phases, the particle sizes, the r.m.s. strains, the lattice parameter changes, etc., have been estimated from Rietveld analysis of X-ray powder diffraction data. It has been found that a higher BPMR exerts more influence on rapid phase transformation. In the m- to c-ZrO₂ phase transformation, no formation of an intermediate tetragonal ZrO₂ phase has been found. The small change in the lattice volume of m-ZrO₂, which is very close to the lattice volume of c-ZrO₂, caused by ball milling may be attributed to this phase change. The formation of the c phase is noticed, in general, after just 1 h of ball milling, and the particle size of the m phase is reduced to a large extent at the first stage of milling and remains almost unchanged with increasing milling time. However, the particle size of the c phase increases with increasing milling time for the samples milled with higher BPMRs (35:1 and 40:1), suggesting that quenching caused by a high impact energy followed by an annealing effect may play a vital role, which is further manifested in the agglomeration of small particles.