Impact of sandblasting on the flexural strength of highly translucent zirconia.

The objective of this study was to assess the influence of alumina sandblasting on the flexural strength of the latest generation of highly translucent yttria partially stabilized dental zirconia (Y-PSZ). Fully-sintered zirconia disk-shaped specimens (14.5-mm diameter; 1.2-mm thickness) of four Y-PSZ zirconia grades (KATANA HT, KATANA STML, KATANA UTML, all Kuraray Noritake; and Zpex Smile, Tosoh) were sandblasted at 0.2 MPa with 50-μm alumina (Al2O3) sand (Kulzer) or left as-sintered (control). For each zirconia grade, the yttria (Y2O3) content was determined using X-ray fluorescence (XRF). Surface roughness was assessed using 3D confocal laser microscopy. Micro-Raman spectroscopy (μ-Raman) and X-ray diffraction (XRD) were used to assess potentially induced residual stresses. Biaxial flexural strength (n = 20) was statistically compared by Weibull analysis. Focused ion beam - scanning electron microscopy (FIB/SEM) was used to observe the subsurface microstructure. Fracture surfaces after biaxial flexural strength testing were observed by SEM. KATANA UTML had the highest Y2O3 content (6 mol%), followed by KATANA STML and Zpex Smile (5 mol%), and KATANA HT (4 mol%). Al2O3-sandblasting significantly increased surface roughness of KATANA UTML and Zpex Smile. μRaman and XRD revealed the presence of residual compressive stress on all Al2O3-sandblasted surfaces. FIB/SEM revealed several sub-surface microcracks in the sandblasted specimens. Weibull analysis revealed that Al2O3-sandblasting increased the characteristic strength of KATANA HT, KATANA STML, whereas it decreased the strength of KATANA UTML. The strength enhancement after Al2O3-sandblasting of KATANA HT was the highest, followed by KATANA STML. For Zpex Smile, the influence was statistically insignificant. The impact of Al2O3-sandblasting on the Weibull modulus was controversial. The strength of zirconia after Al2O3-sandblasting is determined by the balance between microcrack formation (decreased strength) and surface compressive stress build-up (increased strength).