Effects of Sintering Cycle on Zirconia Properties With Various mol%Yttria
Objectives: Compare the effects of sintering time and temperature on grain size, translucency, and flexural strength of different yttria mol% zirconia materials.
Methods: Tosoh zirconia powders Zpex smile, Zpex, TZ-3YB-E, and TZ-3YSB-E were uniaxial dry pressed into blocks, partially sintered at 1000 °C, sectioned into 2mm thick discs, and fully sintered using three different protocols:
1) “conventional”, Vita Zyrcomat T at 1520 °C for 2 h, total 7 h. 2) “fast firing”, Sirona inFire HTC SPEED at 1540 °C for 30 min, total ~1.5 h. 3) “speed firing”, Sirona CEREC Speed Fire at 1579 °C for 5 min, total ~18 min. Contrast ratio was determined using a spectrophotometer X-Rite Ci7600. Specimens were subjected to one of the post treatments:
1) no treatment (control) 2) low temperature degradation (LTD) at 120 °C steam for 1 day 3) LDT 1 week 4) cyclic loading fatigue for 50,000 cycles 5) cyclic load fatigue for 100,000 cycles.
A ball-on-three balls biaxial flexural strength was performed at a cross head speed of 0.5 mm/min. Microstructure was characterized using an SEM and grain size was measured using NIH ImageJ. Data was analyzed using JMP Pro 14.0 at p ≤ 0.05.
Results: Mean grain size for speed sintering Zpex Smile and conventionally sintered TZ-3YSBE were significantly larger compared to other sintering protocols. Speed sintered zirconia shows significantly higher contrast ratio than other firing protocols. Fast sintering groups have significantly higher contrast ratios than conventional sintering. TZ-3YBE had the highest contrast ratio and Zpex had the lowest value. Overall, speed sintering resulted in the highest contrast ratio and conventional sintering the lowest ratio. Speed sintered zirconia had significantly lower flexural strength values.
Conclusions: Sintering protocol had a significant impact on zirconia grain size, translucency, and stability. Speed firing shows lower translucency and flexural strength.