Effect of cooling rate on the microstructural and mechanical properties of speed-sintered monolithic zirconia
Stevan M Čokić1, Jef Vleugels2, Bart Van Meerbeek1
1Department of Oral Health Sciences, BIOMAT-Biomaterials Research Group & UZ Leuven, Dentistry, KU Leuven, Leuven, Belgium.
Objectives:
To evaluate the effect of different speed-sintering cooling rates on the phase composition, microstructure, optical and mechanical properties of monolithic zirconia.
Methods:
Speed-sintered ZirCAD LT (Ivoclar) (LT_1200; 5 min at 1480 °C; oven-door opening and air-cooling: 1200 °C) and Katana STML (Kuraray Noritake) (STML_800; 30 min at 1560 °C; oven-door opening and air-cooling: 800 °C) were compared to speed-sintered ZirCAD LT and Katana STML with a steady cooling rate and oven-door opening at 300 °C (LT_300 and STML_300, respectively). Density was measured by Archimedes' principle; composition by XRF; phase by XRD; grain size by SEM; translucency parameter by spectrophotometry. Vickers hardness, indentation fracture toughness, and biaxial strength were also assessed.
Results:
Regardless of the cooling rate, ZirCAD LT and Katana STML revealed similar density, chemical composition and grain size. Both 3Y-TZP ZirCAD LT_1200/300 zirconia had lower cubic ZrO2-phase content, and lower Y2O3 content in the remaining tetragonal ZrO2 phases compared to 5Y-PSZ Katana STML_800/300 zirconia. In contrast to 3Y-TZP, 5Y-PSZ contained a high amount of metastable t'-phase. TP of ZirCAD LT and Katana STML were not affected by the cooling rate. The hardness and fracture toughness of both 3Y-TZP and 5Y-PSZ were not influenced by cooling rate variations. Steady cooling significantly diminished flexural strength and mechanical reliability of Katana STML_300, while it did not have any effect on the ZirCAD LT zirconia grades.
Significance:
Steady cooling during speed-sintering did not significantly influence the performance of 3Y-TZP but can negatively influence flexural strength and mechanical reliability of 5Y-PSZ zirconia ceramics.
