Effects of surface treatments, LTD aging, and pH on the mechanical behavior of high-translucent zirconia

This study aims to evaluate the effects of low-temperature degradation (LTD) aging and pH variations on the mechanical and surface properties of 4Y-TZP and 6Y-TZP high-translucent zirconia materials subjected to different surface treatments.

A total of 336 Y-TZP specimens (1.2 mm thickness, 12 mm diameter) were prepared and divided into two zirconia groups (4Y-TZP, 6Y-TZP) and four different surface treatment subgroups (glazed, grinding, grinding + polishing, grinding + glazing) (n = 42 per subgroup). Control specimens were stored in distilled water for 28 days (n = 14 per subgroup), while the remaining samples underwent LTD aging in an autoclave for 30 h. After aging, the specimens were immersed in pH 3 (acidic) and pH 9 (basic) solutions for 14 days. The mechanical and surface properties were assessed using biaxial flexural strength testing, Weibull analysis, X-ray diffraction phase analysis, scanning electron microscopy, and digital profilometry for surface roughness measurements.

The findings demonstrated that LTD aging combined with pH 3 exposure significantly affected the mechanical properties and phase transformation of Y-TZP zirconia (p < 0.05). Among the different surface treatments, grinding followed by polishing resulted in the highest biaxial flexural strength and lowest surface roughness, whereas grinding alone led to a significant decrease in mechanical performance. Grinding followed by glazing provided partial protection against surface degradation; however, the combination of LTD aging and pH exposure resulted in an overall reduction in mechanical strength across all groups. The 6Y-TZP group, due to its higher cubic phase content, exhibited greater resistance to LTD but showed lower biaxial flexural strength compared to 4Y-TZP. Additionally, pH 3 conditions induced more severe surface degradation and mechanical weakening, whereas pH 9 conditions resulted in more controlled degradation.

Surface treatments, LTD aging, and pH variations play a critical role in determining the mechanical stability and surface characteristics of Y-TZP zirconia. Grinding followed by polishing is recommended for enhancing restoration durability in clinical applications. Acidic environments (pH 3) can negatively impact mechanical properties, while basic conditions (pH 9) provide comparatively better stability. The optimization of surface treatments and protective clinical strategies is essential to mitigating the adverse effects of LTD and ensuring the long-term success of zirconia restorations.