Mechanical microscopy of the interface between yttria-partially-stabilised zirconia and porcelain in dental prostheses

In recent decades, the high strength, high toughness and appealing aesthetics of Yttria-Partially-Stabilised Zirconia (YPSZ) has made this ceramic the material of choice for dental prostheses. During manufacture YPSZ copings are veneered with porcelain to match the appearance of natural teeth and to reduce wear. However, near-interface chipping of the veneer is observed as a persistent primary failure mode. Recent studies suggest that failure is associated with the mechanical and microstructural state within a few microns of the interface. This insight has provided the motivating driver for this study: to develop and implement new characterisation techniques to improve the understanding of the YPSZ-porcelain interface and thereby reduce failure rates. Microscale characterisation of the interface was performed using energy dispersive X-ray spectroscopy, Raman spectroscopy, X-ray Diffraction (XRD) and transmission electron microscopy. These studies indicated that YPSZ phase variation, elemental composition gradients and distinct microstructural features are present within 10 μm of the interface. Porcelain nanoscale voiding was found at the interface, and small angle neutron scattering confirmed that this is induced by tensile creep. High resolution (microscale) residual stress analysis was performed in YPSZ using XRD and Raman spectroscopy, and in porcelain using a new pair distribution function analysis technique. Cross-validation of these results was performed using improved implementations of the ring-core focused ion beam milling and digital image correlation technique. The variation of Young's modulus, yield strength and fracture toughness were determined using spatially resolved micropillar compression and splitting. Improved evaluation of the single-crystal stiffness of YPSZ was also performed using a new neutron diffraction based technique. The results of this analysis indicate a complex interaction within the first 50 μm of the YPSZ-porcelain interface which leads to a significant reduction in porcelain toughness and an increased propensity to fail at this location.