A Study of Localized Deformation Mechanisms in Y-TZP Dental Ceramics

Objectives: Scanning electron microscopy (SEM) and X-ray diffraction (XRD) are commonly used to characterize the microstructures and crystalline phases in dental zirconia, but they do not provide a complete picture of microstructural level changes during processing and fracture. While XRD works well for characterizing bulk phase transformations such as aging processes, it is typically unable to capture small localized phase transformations such as those taking place around advancing cracks. The objective of the present study was to examine localized crack propagation deformation mechanisms in dental zirconia using electron backscattered diffraction (EBSD). We hypothesized that localized tetragonal-to-monoclinic transformation occurs in a continuous envelope around advancing cracks in 3Y-TZP.
Methods: 4 specimens of 3Y-TZP (ZirCAD LT) were sectioned from a CAD/CAM block and sintered using the manufacturer’s recommended 9h and 50min firing procedure. Sintered specimens were rotary polished with successive diamond lapping films to a 0.1µm finish followed by vibratory polishing to a 0.05µm finish for 8h. Polished specimens were indented using a Vickers’ indenter at 150N loads to introduce radial surface cracks. Indented specimens were sputter-coated with 4nm of carbon to reduce surface charging for EBSD surface scans. Localized microstructure areas around the radial crack paths were analyzed using EBSD. Grain orientation, crystalline phase, and grain size, were generated from these EBSD datasets.
Results: Examination of microstructure areas surrounding radial cracks revealed discontinuous monoclinic transformation around the advancing cracks. Moreover, monoclinic transformation was not limited to the largest tetragonal grains along the crack paths as previously assumed by many researchers. Finally, each area showing crack bridging, revealed at least some monoclinic transformation in the bridging zone.
Conclusions: EBSD analyses were successful in revealing localized deformation around the crack paths in 3Y-TZP. The study hypothesis was rejected as EBSD analyses showed that tetragonal-to-monoclinic phase transformation was discontinuous around the advancing cracks.