Effect of surface finishing and thickness on the translucency of zirconia dental ceramics

Abstract The effects of different surface finishing procedures on the translucency of yttria stabilized zirconia ceramics (ZrO2) were evaluated. ZrO2-3mol.%Y2O3 (designed Zr3) or ZrO2-5mol.%Y2O3(Zr5) specimens with thickness varying between 0.5 and 1.5 mm were obtained by sintering at 1500 °C for 2 h. Surface finishing of the sintered specimens was done in three distinct manners: polishing with diamond pastes, blasting with Al2O3 particles and blasting with Al2O3–SiO2 particles, according to the preparation protocol for dental prostheses. These sample groups were characterized by X-Ray diffraction analysis, scanning electron microscopy, surface roughness and spectrophotometry. The sintered ceramics exhibited relative densities higher than 99% and were composed of 68.5% and 31.5%of tetragonal (t)-ZrO2 and cubic(c)-ZrO2 for composition Zr3 and 25.2% and 74.8% for composition Zr5, respectively. Furthermore, 1.5 to 1.8% of monoclinic (m)-ZrO2, has been found in the blasted surfaces. The average grain size varied from 0.5 μm for Zr3 to 1.45 μm for the Zr5 samples. After polishing, all samples presented a surface roughness Raof about 0.05 μm, while blasting resulted in an increased roughness ranging between 1.24 μm and 1.49 μm.Samples rich in (c)-ZrO2 phase (Zr5) are more translucent than samples rich in (t)-ZrO2 (Zr3) because of their larger grain size and because the cubic phase is less anisotropic than (t)-ZrO2. Furthermore, the translucency of thinner samples is more affected by abrasive blasting because they also present the highly anisotropic monoclinic(m)-ZrO2 phase and, therefore, the reduction of translucency is more pronounced. Parameters such as grain size, crystalline phase composition, porosity and grain boundary density are used to explain the phenomena involved in the differences of translucency of these materials.