Mechanical reliability, fatigue strength and survival analysis of new polycrystalline translucent zirconia ceramics for monolithic restorations

This study characterized the mechanical properties (static and under fatigue), the crystalline microstructure (monoclinic - m, tetragonal - t and cubic - c phase contents) and the surface topography of three yttrium-stabilized zirconia (YSZ) materials with different translucent properties, before and after aging in an autoclave (low temperature degradation). Disc-shaped specimens were produced from second generation (Katana ML/HT - high-translucent) and third generations (Katana STML - super-translucent and UTML - ultra-translucent) YSZ ceramics (Kuraray Noritake Dental Inc.), following ISO 6872-2015 guidelines for biaxial flexural strength testing (final dimensions: 15 mm in diameter and 1.2 ± 0.2 mm in thickness), and then subjected to the respective tests and analyses. ML was mainly composed of tetragonal crystals, while STML and UTML presented cubic content. Aging increased the monoclinic content for ML and did not affect STML and UTML. Topographical analysis highlights different grain sizes on the ceramic surface (UTML > STML > ML) and aging had no effect on this outcome. Weibull analysis showed the highest characteristic strength for ML both before and after aging, and statistically similar Weibull moduli for all groups. ML material also obtained the highest survival rates (ML > STML > UTML) for both fatigue strength and number of cycles to failure. All fractures originated from surface defects on the tensile side. Third generation zirconia (Katana STML and UTML) are fully stabilized materials (with tetragonal and cubic crystals), being totally inert to the autoclave aging, and presented lower mechanical properties than the second-generation zirconia (Katana ML - metastable zirconia).