Methods: Two commercial 3Y-TZP granules, undoped and doped with 0.202% Fe2O3, respectively, were dry pressed one above the other, followed by cold isostatic pressing and pressureless sintering (1450oC) to prepare bi-colored and pre-sintered zirconia blanks. The sintering process was investigatedby in-situ dilatometer that also used to calculate thermal expansion coefficients (TEC). Microstructure and Vikers hardness of bi-colored bars were characterized. Four-point flexure strength was measured with one-step pressed white/yellow zirconia bars as control (n = 10 per group). Data was analyzed by One-way ANOVA (α=0.05).
Results: ~1mm thickness color gradient zone (CGZ) with light yellow color was formed at the pressed interface between white and yellow sides, due to diffusion of Fe2O3. No excessive residual stresses concentrated in CGZ since two sides had similar shrinkage (~25%) and TCE (~10.5x10-6/K, 500oC), although yellow-side with Fe2O3 doping sintered slightly rapid. No obvious microstructrual difference or defects was found in CGZ, while it had similar grain size, ~350nm, and Vickers hardness, ~13.3 GPa, with the sides (p > 0.05). The flexural strength of bi-colored zirconia, 746±160 MPa, was noticeably lower than the single-colored references (p < 0.05). Fractographic analysis revealed that more fracture (60%) was initiated at CGZfor bi-colored bars, where large voids, critical agglomerates and irregular porous grainswere observed; while less occurred at the medium in single-colored groups.
Conclusion: The feasibility of process might be convinced since the acceptable microstructure and mechanical properties of bi-colored zirconia. CGZ could natural-mimicking the gradual color between enamel and dentin and might be tailored by controlling doping diffusion. CGZ is a weak region, probably not due to thermal residual stresses but the processing defects, which is expected to be minimized.