Recent advances in additive manufacturing (AM) allowed new approaches to design dental restorations from zirconia ceramics. Stereolithography-based AM (SLA) can produce precise and dense objects, and unlike CAD-CAM milling of dry-pressed and presintered zirconia blocks, less or no waste material is left behind. The performance of SLA is near to achieving the level of conventionally used methods. However, dental surface treatments, i.e., airborne particle abrasion (APA) or regeneration firing (RF), may significantly alter SLA-fabricated zirconia ceramics' surface and bulk integrity, which may be more pronounced for less tough, translucent zirconia ceramics. Therefore, the aim was to evaluate and compare the effects of APA and RF on the mechanical properties of different zirconia ceramics fabricated with SLA and dry-pressing (DP).
Disk-shaped specimens (n=90 per group) were fabricated from 3 %mol. (3Y) and 5 %mol. (5Y) yttria-zirconia by SLA (CeraFab 7500; Lithoz GmbH, Vienna, Austria; lateral resolution of 40 μm and layer thickness of 25 μm) and uniaxially DP (147 MPa, 30 s) that served as control. The top surface of specimens (n=60) was treated by APA with 50 μm Al2O3 at 2 bars. Half of the APA specimens were treated with regeneration firing (RF; 1000°C, 15 min). Control specimens (n=30) were left in an as-sintered (AS) state. Biaxial flexural strength was determined using the ball-on-three-balls method coupled with Weibull analysis and fractography (SEM). XRD and FIB-SEM determined crystal phases and depth of transformation zone (TZD), respectively. ANOVA with Tukey's post hoc and t-test was used (p<0.05).
In relation to DP zirconia, SLA zirconia exhibited lower characteristic strength (σ_?) and reliability (m), regardless of zirconia type (3Y, 5Y). The APA increased σ_? of 3Y, where surface compressive stresses created by t-m transformation toughening were observed, and decreased σ_? of 5Y material, where mechanical strength, in the absence of t-m toughening, was merely related to the APA-introduced flaws. Reliability (m) was increased for 5Y. The RF reversed the effects of APA, regardless of zirconia type; however, m was increased for both materials compared to AS. No differences between SLA and DP were noted regarding crystal phases and TZD. The prevailing critical flaws for SLA and DP were process-related; the size of flaws was considerably larger in SLA. For the SLA material, feedstock-related, printing-related, and debindment-related critical flaws could be differentiated.
SLA zirconia presented mechanically inferiorly compared to DP, still reaching clinically-acceptable strength, whereas APA and RF effects did not differ between SLA and DP zirconia. The main observed defects were process-related.