Transient thermal stresses developed during speed sintering of 3 mol% yttria-stabilized tetragonal zirconia polycrystals

Objectives: To investigate transient thermal stresses that developed in 3Y-TZP green compacts during speed sintering.Methods: A total of 312 disc-shaped green compacts (o17.1 x1, 1.5, 2, 2.5, 3 mm) were cold-isostatically pressed from 3Y-TZP powder (Zpex, Tosoh Corp.) for speed sintering studies as well as compositional analysis and biaxial flexural strength measurements (both at room temperature and following heating at 90 degrees C/min to 500 degrees C). Flexural strength was determined using the piston-on-3-ball method. Phase assemblies were analyzed using the X-ray diffraction method. Effects of heating/cooling rates on transient stresses were investigated by conducting definitive sintering studies to determine the threshold for fracture. Finite element analysis (FEA) was used to validate the experimental findings using measured thermomechanical properties.Results: The bulk and relative density of the green compact were 2.95 +/- 0.03 g/cm3 and 48.52% +/- 0.45%. The flexural strength was drastically decreased from 10.3 +/- 0.4 MPa to 1.09 +/- 0.07 MPa following heating at 90 degrees C/min to 500 degrees C. The monoclinic and tetragonal contents were 54.9% and 45.1%, respectively. The threshold for fracture was located at 500 degrees C during the first heating stage with a 90 degrees C/min heating rate in specimens of 2.5 mm thickness or greater. No fractures occurred in the second heating stage and cooling phase. The FEA estimated that the principal transient tensile stress was similar to 1.14 MPa at 500 degrees C during the heating phase, which exceeded the corresponding flexural strength (1.09 +/- 0.07 MPa).ignificance: Advanced FEA methods are an accurate and efficient tool to analyze the history of transient stresses during sintering of ceramic dental restorations.