Effect of Coloring on Mechanical Properties of Dental Zirconia

Pre-sintered blocks of three kinds of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) and one ceria-stabilized tetragonal zirconia polycrystal (Ce-TZP)/alumina nanocomposite were stained with six kinds of coloring liquid (three ivory, two violet, and one pink). The crystal phases of the baking powder of the coloring liquids fired at 1100 degrees C for 30 min were characterized by X-ray diffractometry (XRD) and their constituent elements were detected by wavelength dispersive X-ray spectroscopy (WDS). The three ivory liquids contained mainly Fe ions, the two violet liquids mainly contained Co ions, and the pink liquid contained Er and Nd ions. After the final sintering, the color differences (Delta E) were determined with a chroma meter. Three-point flexural strengths were determined at a span length of 16 mm according to ISO 6872. Fracture toughness was determined by an indentation method in terms of Palmqvist cracks. The color difference of the nanocomposite was smaller than those of three Y-TZPs due to high light scattering. The three ivory liquids for the nanocomposite led to small Delta E values. The sintered zirconia stained with the pink liquid containing Er and Nd ions showed significant decreases in flexural strength and fracture toughness. An XRD study revealed that most of the crystal phase of the zirconia stained with Er and Nd was cubic. In contrast, the three ivory and two violet liquids led to no significant change in the flexural strength and fracture toughness of the zirconia.