Effect of Grain Refinement on Strength and Reliability of 3Y-TZP

Objectives: Previous work has shown that heat treatment of zirconia after grinding or air abrasion leads to grain refinement by recrystallization, which may be desirable for dental implant surfaces. Our goal was to investigate the effect of grain refinement by recrystallization heat treatment on the flexural strength and reliability of 3Y-TZP.
Methods: Disc-shaped specimens were sectioned from 3Y-TZP blanks, sintered at 1450°C/2h and assigned to 7 groups (n=20).One group was left as-sintered as control (AS). Three groups were either ground to 600 grit (G), ground and recrystallized at 1200°C/0 min. (GR0) or 1200°C/15 min. (GR15). Three groups were either air-abraded (AA), air abraded and recrystallized at 1200°C/0 min. (AAR0) or 1200°C/15 min. (AAR15). The mean grain size was measured from AFM images.The biaxial flexural strength was measured in water according to ISO standard 6872. Results were analyzed by ANOVA and Tukey’s test as well as Weibull statistics. In addition, specimens (n=3 per group) were aged for 5h in an autoclave. Crystalline phases were analyzed by grazing incidence x-ray diffraction (GIXRD).
Results: GIXRD revealed the only presence of rhombohedral phase at the surface after grinding. Air abrasion led to peak broadening and a small amount of monoclinic phase (m-phase). Recrystallization heat treatment led to a significant decrease in grain size, from 0.52±0.06μm to 0.125±0.02μm and suppressed the formation of m-phase after aging for 5h, regardless of duration of heat treatment. The mean biaxial flexural strength was significantly higher (p<0.05) for group G (1644.4±150.1MPa), followed by AA (1593.7±122.4MPa), GR15 (1496.9±175.2MPa), GR0 (1429.3±142.7MPa), compared to AS control (1256.9±127.4MPa). Groups AA, AAR0 and AAR15 exhibited the highest Weibull moduli, respectively 15.62, 15.38 and 13.46, compared AS control (10.61).
Conclusions: Heat treatment at 1200°C promoted grain refinement and prevented aging, regardless of duration or type of surface treatment. Surface grinding was more efficient in increasing the strength than air abrasion. However, air abrasion treatments were associated with the highest reliability.