Mapping the tetragonal to monoclinic phase transformation in zirconia core dental crowns.

Chipping failures observed clinically in bilayer systems of porcelain and zirconia restorations should be coupled with a monoclinic to tetragonal phase transformation in the zirconia layer due to the high compressive stress. Phase transformations were mapped using 2D micro X-ray diffraction of 1083 frames at 100μm×100μm spacing automatically positioned along the core layer of a sectioned fractured crown. Yttria-zirconia tetragonal phase transformations to monoclinic zirconia and monoclinic yttria were observed, mostly at the impacted area. A simple map of (101) tetragonal d-spacing strain reveals stress relaxation during phase transformation was detected at inner section of lingual side, because the initial state of compressive residual stress assists this phase transformation at the inter section of lingual side of the core while initial tensile stress at the outer sides under the veneer relaxes under compression and initially prevents phase transformation. This study implements an experimental method to map the phase transformation, after applying local compressive load until fracture. Such fractures resemble clinically observed chipping failure.