Understanding transformation toughening in tetragonal and monoclinic mixed-phase yttria-stabilized zirconia

Abstract

Yttria-stabilized zirconia powders with varying yttria from 0 to 4.5 mol% were synthesized via the sol-gel method and subsequently calcinated at 1350 °C. The calcined powders were then consolidated using spark plasma sintering at 1600 °C, achieving relative densities exceeding 98% without significant coarsening of the nanocrystalline grains. Structural characterization through X-ray diffraction confirmed the partial to complete stabilization of the tetragonal phase with increasing yttria content. Microhardness increased from 5 to 15 GPa with yttria content up to 2 mol%. Further, indentation method indicated a peak fracture toughness of 18.2 MPa m1/2 for the 1 mol% yttria sample. Electron backscatter diffraction and Raman spectroscopy analyses demonstrate that the enhanced local fracture toughness is attributed to the optimized metastability of the tetragonal phase in the 1 mol% yttria sample, where the tetragonal grain size approaches the “critical grain size”. However, the bulk fracture toughness measured using a three-point bending test was found to be low (∼3.18 MPa m1/2), comparable to that of monoclinic zirconia. This study demonstrates that superior indentation fracture toughness does not necessarily translate to high bulk toughness when a significant fraction of the monoclinic phase is present, as it dominates the overall response in a two-phase microstructure. © 2026 Elsevier Ltd and Techna Group S.r.l. All rights are reserved, including those for text and data mining, AI training, and similar technologies.