EBSD-Based Strain Mapping Across Crack Process Zones in Dental Zirconia

Objectives: Electron-backscattered diffraction (EBSD) analyses capture grain orientation and crystalline phase information for each pixel within an SEM field of view. Since information is captured for each pixel within individual grains, residual strain maps of localized areas in zirconia microstructures are possible. Both the tetragonal and cubic phases of zirconia have known pseudo-symmetry issues that complicate traditional Hough-based EBSD indexing. Our research team is the first to apply innovative forward-based spherical indexing to correctly index the zirconia phases. The objective of this study was to utilize EBSD spherical indexing to study strain development within crack process zones in 3Y-TZP.
Methods: 3Y-TZP (ZirCAD LT) specimens (n=4) were sintered using the manufacturer’s recommended 9 hour and 50 minute firing cycle. Specimens were then polished to a 0.05 µm finish using very low loads to minimize phase transformation. Polished surfaces were then indented using a Vickers’ indenter with 150N loads to generate radial surface cracks. After sputter-coating with 4nm of carbon to reduce charging, EBSD scans were performed on representative areas surrounding the radial crack paths. EBSD phase and Kernel Average Mismatch (KAM) residual strain maps were produced for each microstructural area.
Results: Increased levels of residual strain were shown in the tetragonal grains surrounding each radial crack path. Grains along the crack paths that had experienced tetragonal-to-monoclinic transformations still showed some increased residual strain. Evidence of crack bridging with monoclinic transformation was shown in 3 microstructural areas, and in each case the highest residual strain levels were shown at the crack tips on the sides of the bridging area.
Conclusions: EBSD analyses enabled residual strain mapping in the crack process zones of 3Y-TZP zirconia for the very first time. This new laboratory tool shows much promise for studying crack propagation mechanisms in future graded and non-graded dental zirconia.