Effects of repetitive firings on mechanical and phase formation changes in zirconia produced by CAD-CAM and 3D printing

Statement of problem: Although 3-dimensionally (3D) printed zirconia has been developed as an alternative to milled computer-aided design and computer-aided manufactured (CAD-CAM) zirconia, knowledge on the material's mechanical and phase properties, as well as how these properties change after repeated firing is lacking. Purpose: The purpose of this in vitro study was to determine the effects of repetitive firings on the phase transformation and mechanical properties of zirconia materials produced by 2 different techniques. Material and methods: A total of 104 rectangular prisms (5×5×1 mm) and 8 disks (Ø6×1.35 mm) of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) zirconia produced by CAD-CAM and 3D printing were used. Specimens from groups were divided into 4 groups (n = 13) with a different number of glaze firings (1-4) by simple randomization. After the firings, a surface roughness evaluation, Vickers hardness evaluation, X-ray diffraction and Rietveld analysis, environmental scanning electron micrograph analysis, and 3D surface topography analysis were performed. Data were analyzed with analysis of variance (2-way ANOVA) (α=.05). Results: Repeated firing did not affect the surface roughness of the specimens in either group (P>.05), while the surface hardness was significantly affected (P<.05). In XRD analysis, the specimens produced by the 2 techniques had peaks at the same degrees and number of firings, whereas in the Rietveld analysis, the tetragonal phase ratio of CAD-CAM zirconia specimens increased with the number of firings, and the tetragonal phase ratio of 3D zirconia specimens decreased after 4 firings. Conclusions: Despite exhibiting hardness and phase characteristics comparable with those of the CAD-CAM zirconia specimens, 3D zirconia specimens had a smoother surface. After the fourth firing, the tetragonal phase ratio began to decline.