Effect of Surface Modification on In-Depth Transformations and Flexural Strength of Zirconia Ceramics

Purpose Chairside surface adjustments of zirconia dental restorations enhance the toughening stress-induced tetragonal-to-monoclinic phase transformation and domain reorientation by ferro-elastic domain switching (FDS), but also trigger subsurface damage, which could compromise long-term clinical performance. The purpose of this study was to assess the depth of phase transformation, associated FDS, and flexural strength of dental zirconia (BruxZir HT 2.0), after chairside surface treatments. Materials and Methods Square specimens were sectioned from CAD/CAM blocks and sintered according to manufacturer's recommendations (n = 30). They were left as-sintered (AS; control), air abraded with fine (AAF) or coarse (AAC) alumina particles, ground (G) or ground and polished (GP). Roughness was measured by profilometry. Crystalline phases were investigated by grazing incidence X-ray diffraction (GIXRD) (n = 3). GIXRD data were fit using semi-log regression protocols to assess transformation depth and extent of FDS. The mean biaxial flexural strength was measured according to ISO 6872. Subsurface damage was assessed from SEM images using a bonded polished interface configuration. Flaw distribution was assessed by Weibull analysis. Results were analyzed by Kruskal-Wallis with Tukey's adjustment for multiple comparisons (p < 0.05). Results Air-abraded and ground groups exhibited higher mean surface roughness than control. AAF group exhibited the highest flexural strength (1662.6 +/- 202.6 MPa) with flaw size (5.9 +/- 1.8 mu m) smaller than transformation (14.5 +/- 1.2 mu m) or FDS depth (19.3 +/- 1.1 mu m), followed by GP group (1567.2 +/- 209.7 MPa) with smallest FDS depth (9.3 +/- 2.0 mu m) and flaw size (2.6 +/- 1.8 mu m), but without m-phase. AAC group (1371.4 +/- 147.6 MPa) had the largest flaw size (40.3 +/- 20.3 mu m), transformation depth (47.2 +/- 3.0 mu m) and FDS depth (41.2 +/- 2.2 mu m). G group (1357.0 +/- 196.7 MPa) had the smallest transformation depth (8.6 +/- 1.5 mu m), and mean FDS depth (19.8 +/- 3.7 mu m) and flaw size (18.6 +/- 3.1 mu m). AAC and AAF exhibited the highest Weibull modulus (11.2 +/- 0.4 and 9.8 +/- 0.3 mu m, respectively). Conclusions Variations in mean biaxial flexural strength were explained by the balance between the depth of toughening mechanisms (phase transformation and FDS) and subsurface damage. AAF and GP groups were the most efficient surface adjustments in promoting the highest mean biaxial flexural strength.