Browsing by Author "Alshahrani, Abdulaziz"

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OPTIMIZING SPEED SINTERING PARAMETERS FOR YTTRIA-STABILIZED ZIRCONIA
(University of Pennsylvania, 2024) Alshahrani, Abdulaziz; Zhang, Yu
Sintering zirconia-based ceramics is a time-consuming process that uses slow heating and cooling rates. To fulfill the requirement of chair-side production of zirconia, speed and high-speed sintering technologies have emerged. However, the impact of these recent technologies on the thermal (transient stresses), physical, optical, and mechanical properties of yttria-stabilized zirconia (YSZ) still needs to be investigated. Therefore, a total of 750 disc-shaped specimens were pressed from zirconia powder (Zpex series, Tosoh Corp., Japan). Density was measured using the Archimedes method; the phase contents were determined by XRD; translucency was evaluated by a spectrophotometer on a black and white background; flexural strength and fatigue resistance were assessed by the biaxial flexure method; and the microstructure was examined by SEM imaging. We observed that transient thermal stresses were developed during sintering as a result of thermal gradients due to the fast heating and cooling rates adopted by speed firing. Finite element analysis (FEA) was utilized to quantify the magnitude of transient thermal stress. These significant observations highlighted the importance of pre-sintering conditions, especially if the pre-sintered zirconia blocks and discs were intended for speed sintering. Further, our findings showed that the currently recommended speed and high-speed sintering protocols of YSZ resulted in incomplete densification due to the brief dwell time of 5–16 min compared to 120 min in conventional sintering. Subsequently, the entrapped pores resulted in light scattering and reduced optical translucency. Interestingly, residual pores did not affect the flexural strength. The speed sintering protocols for 3–5 mol% YSZ were optimized by proposing higher sintering temperatures, 1550°C and 1580°C, and longer dwelling times, 40 min and 60 min. As a result, SS-1 (1550°C/40 min) for 3YSZ, SS-3 (1580°C/40 min) for 4YSZ, and SS-4 (1580°C/60 min) for 5YSZ was the optimal speed sintering conditions for those compositions, which led to significantly lower properties than the current speed and high-speed sintered YSZ and comparable to the gold standard “conventional sintering.” Finally, we demonstrated that the selected optimal speed sintering protocol for 5YSZ is more fatigue-resistant and exhibits better long-term mechanical stability.
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