Structural, Mechanical, Imaging and in Vitro Evaluation of the Combined Effect of Gd³⁺ and Dy³⁺ in the ZrO₂–SiO₂ Binary System

Mechanical strength and biocompatibility are considered the main prerequisites for materials in total hip replacement or joint prosthesis. Noninvasive surgical procedures are necessary to monitor the performance of a medical device in vivo after implantation. To this aim, simultaneous Gd³⁺ and Dy³⁺ additions to the ZrO₂–SiO₂ binary system were investigated. The results demonstrate the effective role of Gd³⁺ and Dy³⁺ to maintain the structural and mechanical stability of cubic zirconia (<i>c</i>-ZrO₂) up to 1400 °C, through their occupancy of ZrO₂ lattice sites. A gradual tetragonal to cubic zirconia (<i>t</i>-ZrO₂ → <i>c</i>-ZrO₂) phase transition is also observed that is dependent on the Gd³⁺ and Dy³⁺ content in the ZrO₂–SiO₂. The crystallization of either ZrSiO₄ or SiO₂ at elevated temperatures is delayed by the enhanced thermal energy consumed by the excess inclusion of Gd³⁺ and Dy³⁺ at <i>c</i>-ZrO₂ lattice. The addition of Gd³⁺ and Dy³⁺ leads to an increase in the density, elastic modulus, hardness, and toughness above that of unmodified ZrO₂–SiO₂. The multimodal imaging contrast enhancement of the Gd³⁺ and Dy³⁺ combinations were revealed through magnetic resonance imaging and computed tomography contrast imaging tests. Biocompatibility of the Gd³⁺ and Dy³⁺ dual-doped ZrO₂–SiO₂ systems was verified through in vitro biological studies