New Speed-Sintered CaO-Zirconia for Strong, Tough, Translucent Restorations

The demand for rapid, reliable, and esthetic ceramic restorations continues to drive innovation in dental materials and chairside manufacturing technology. However, conventional yttria-stabilized zirconia presents an inherent trade-off between mechanical and optical properties, with moderate fracture toughness ≤5 MPa·m^1/2. This study investigates the feasibility of chairside speed-sintering of a 4.5 mol% CaO-stabilized tetragonal zirconia polycrystal (4.5Ca-TZP), focusing on its microstructure, mechanical reliability, translucency, and aging resistance. Nano-sized 4.5Ca-TZP powders were compacted and speed-sintered within 60 min at 1,250 °C to 1,350 °C. Four sintering groups were evaluated (n = 20/group for density; n = 1 for grain size; n = 10 for hardness; n = 3 for Rietveld refinement; n = 5 for fracture toughness, translucency, and aging; n = 30 for biaxial strength and Weibull analysis) with statistical differences set at α = 0.05. Speed-sintering gave rise to fully dense ceramics (≥99% relative density) with a homogeneous, fine microstructure (<200 nm) and negligible monoclinic content (≤2 vol%). The ceramics demonstrated excellent mechanical reliability, exhibiting a characteristic strength of ≥1.1 GPa and a Weibull modulus ≥11. Hardness slightly decreased with increasing temperature, whereas translucency reached its maximum (≈22) at 1,250 °C. Indentation testing revealed the absence of radial cracks in specimens sintered above 1,300 °C. No tetragonal-to-monoclinic transformation occurred after 20 h of hydrothermal aging at 134 °C, demonstrating aging resistance. Compared with conventionally sintered 4.5Ca-TZP, speed-sintered ceramics showed comparable translucency and strength with enhanced mechanical reliability but significantly lower fracture toughness of >5 MPa·m^1/2. Overall, this work demonstrates that 4.5Ca-TZP can be speed-sintered to achieve fully dense zirconia with performance exceeding that of conventional 3Y-TZP, providing a tougher alternative zirconia restoration. A computer-aided design/computer-aided manufacturing-milled crown fabricated as a proof of concept showed isotropic shrinkage, supporting its potential for chairside application, while further investigation of long-term resistance, large-scale manufacturing, and dimension limitations is required to confirm clinical applicability.