Publication date: Available online 12 August 2017
Author(s): Dušan Bučevac, Tomaž Kosmač, Andraž Kocjan
The yttrium-segregation-dependent phase partitioning and residual stress development that influence both the aging and the fracture behaviour in 3Y-TZP bioceramics were studied by sintering alumina-free 3Y-TZP, varying the sintering temperature and the time, to yield ceramics with identical grain size distributions, but with different phase compositions. The structure and stability of the resulting tetragonal phases, in the form of transformable, yttria-lean t-ZrO2 (YLZ) and non-transformable, yttria-rich t”-ZrO2 and/or t’-ZrO2 (YRZ), were studied by X-ray diffraction (XRD) and focused ion beam scanning electron microscopy (FIB-SEM). The accelerated aging kinetics was fitted to the Mehl-Avrami-Johnson equation. The specimen sintered at the lowest sintering temperature but with the longest dwell time contained the smallest and the largest concentrations of yttria in the YLZ and YRZ phases, respectively, as well as the largest amount of YRZ. As a consequence, it exhibited the fastest linear aging kinetics accompanied by more extensive micro-cracking of the transformed layer, as well as largest amount of intergranular fracture and the greatest resistance to fracture. These properties were ascribed to the increased transformability of the YLZ phase and the greatest propensity of the YRZ phase to relax the accumulated residual stresses during transformation (tetragonal to monoclinic, t-m) manifested as a ∼2.4% unit-cell volume increase. The observed relaxation provides additional understanding of the t–m transformation mechanism, which governs both the aging and fracture behaviour in 3Y-TZP.Statement of significanceA novel approach to understanding the effect of yttrium segregation on t–m transformation of 3Y-TZP zirconia bioceramics is presented. Carefully designed sintering strategy facilitated fabrication of ceramics with identical grain size distributions but with different yttrium concentrations. The influence of phase partitioning on stability and structure of transformable yttria-lean tetragonal phase (YLZ) and non-transformable yttria-rich phases (YRZ; t”– and t’–prime) and on the formation of residual stresses in YRZ were investigated. It is shown that YRZ phases are under compressive stresses in YLZ matrix, since a systematic relaxation after ageing was observed and explained for the first time. It puts additional perspective on the understanding of the t–m transformation mechanism ultimately governing both the ageing and fracture behaviour in 3Y-TZP.
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