[Comparative study on fracture toughness of digital light processing three-dimensional printing zirconia and milled zirconia].

Objective: To investigate the influence of digital light processing (DLP) and computer numerical controlmilling (CNC) on the mechanical behavior of zirconia. Methods: Prepared DLP samples (experimental group, n=52) and CNC samples (control group, n=52) with 12 samples in each group were randomly selected using random number table to measure density, grain size and crystal phase composition. According to the different methods fracture toughness test, the samples were divided into indentation method group (IM) and single-edge-V-notch-beam group (SEVNB), with 30 DLP and 30 CNC samples in IM group, 10 DLP and 10 CNC samples in SEVNB group. The IM group was tested under three different loads (49.03 N, 98.07 N, 196.10 N), there were 10 samples for each load and each sample was tested at 15 points, and the load with the ratio of crack length to indentation diagonal length greater than 2.5 was selected as the indentation load to calculate its IM fracture toughness. At the same time, the SEVNB group was tested with four point bending test to record the maximum load at the time of fracture and calculate the SEVNB fracture toughness. Finally, the indentation and fracture surface were observed using optical microscope and scanning electron microscope, and the results of DLP group and CNC group were further compared to explore the difference in fracture mechanism. Results: The microstructure of DLP and CNC zirconia was basically the same, the density of DLP group was (6.020±0.021) g/cm3, the grain size was (0.603±0.033) μm; the density of CNC group was (6.038±0.012) g/cm3, the grain size was (0.591±0.033) μm. Both groups were composed of tetragonal zirconia. The load of 196.10 N was chosen as the indentation load for two groups to calculate the IM fracture toughness. In terms of fracture toughness, there was no significant difference between the two groups (P>0.05). Scanning electron microscope images of fracture surface showed the intergranular fracture was the leading fracture mode of two groups. The IM and SEVNB fracture toughness of DLP zirconia were (6.111±0.179) MPa·m1/2 and (7.221±0.809) MPa·m1/2, respectively. The IM and SEVNB fracture toughness of CNC zirconia were (6.126±0.383) MPa·m1/2 and (7.408±0.533) MPa·m1/2, respectively. Conclusions: The microstructure of DLP and CNC zirconia is almost the same, and there is little difference in the fracture toughness of zirconia between two processing technologies.