Flexural and structural behavior of additively manufactured zirconia: Influence of firing process and build orientation

Additive manufacturing of zirconia is gaining popularity in dentistry; however, knowledge of the influence of build orientation and firing process on its mechanical properties and structural behavior remains limited.

The purpose of this in vitro study was to compare the effect of firing process and build orientation on the flexural strength (FS) and structural characteristics of additively manufactured 3% mol yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) with subtractively manufactured 3Y-TZP and 4Y-TZP.

A total of 192 bar-shaped (17×4×1.5 mm) and 16 disk-shaped (Ø10×1.5 mm) specimens were manufactured either additively with a 3Y-TZP slurry (INNI Cera) or subtractively with presintered 3Y-TZP (IPS e.max ZirCAD LT [SM-LT]) or 4Y-TZP (IPS e.max ZirCAD MT [SM-MT]) zirconia disks. The additively manufactured specimens were categorized based on build orientation (0-degree [AMH], 45-degree [AMD], and 90-degree [AMV]) and firing process (combined [1-step] or separate [2-step] debinding and sintering cycle). All specimens were divided into 2 groups (n=12 for bar-shaped and n=1 for disk-shaped specimens) of aged (thermocycling for 10 000 cycles between 5 °C and 55 °C) and nonaged. Bar-shaped specimens were tested for FS, while disk-shaped specimens were used for structural characterization using X-ray diffraction (XRD) and scanning electron microscope (SEM) imaging. The FS data were analyzed with general linear model analysis and Bonferroni-corrected post hoc tests (α=.05).

The interaction between manufacturing protocol and aging condition affected the FS values (P<.001). Among the nonaged specimens, SM-LT led to higher FS than AMD-2 and AMV-1, while AMH-1 led to higher FS than AMV-1 (P≤.007). Among the aged specimens, AMV resulted in the lowest FS, regardless of the firing process (P≤.034). Aging reduced the FS of AMV-2 (P<.001). All subgroups showed a dominant tetragonal phase with a detectable monoclinic peak at 51.2-degree, with AMD exhibiting lower peak intensity than AMH and AMV, and SM-LT showing higher tetragonal peak intensity than SM-MT; aging increased localized crystallinity in all subgroups, as evidenced by higher XRD peak intensities. SEM analysis showed that AM specimens exhibited characteristic delineation and residual pores, whereas SM specimens displayed linear milling marks from burs and a higher concentration of monoclinic-phase crystals.

At 0-degree orientation, the tested 3Y-TZP slurry demonstrated load-bearing capacity comparable with that of subtractively manufactured 3Y-TZP and 4Y-TZP, regardless of the firing process. However, SEM analysis of AM specimens revealed residual pores, and SM specimens showed a higher concentration of monoclinic-phase crystals.