Effect of artificial aging on fracture toughness and hardness of 3D‐printed and milled 3Y‐TZP zirconia

Abstract Purpose This study aimed to evaluate the impact of artificial aging on the fracture toughness and hardness of three‐dimensional (3D)‐printed and computer‐aided design and computer‐aided manufacturing (CAD‐CAM) milled 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP). Materials and Methods Forty bar‐shaped specimens (45 × 4 × 3 mm) were prepared using two manufacturing technologies: 3D printing (LithaCon 3Y 210, Lithoz GmbH, Vienna, Austria; n = 20) and milling (Initial Zirconia ST, GC, Japan; n = 20) of 3Y‐TZP. The chevron‐notch beam method was used to assess the fracture toughness according to ISO 24370. Specimens from each 3Y‐TZP group were divided into two subgroups ( n = 10) based on the artificial aging process (autoclaving): nonaged and aged. Nonaged specimens were stored at room temperature, while aged specimens underwent autoclave aging at 134°C under 2 bar‐pressure for 5 h. Subsequently, the specimens were immersed in absolute 99% ethanol using an ultrasonic cleaner for 5 min. Each specimen was preloaded by subjecting it to a 4‐point loading test, with a force of up to 200 N applied for three cycles. Further 4‐point loading was conducted at a rate of 0.5 mm/min under controlled temperature and humidity conditions until fracture occurred. The maximum force ( F max ) was recorded and the chevron notch was examined at 30 × magnification under an optical microscope for measurements before the fracture toughness ( K Ic ) was calculated. Microhardness testing was also performed to measure the Vickers hardness number (VHN). A scanning electron microscope (SEM) coupled with an energy dispersive X‐ray unit (EDX) was used to examine surface topography and chemical composition. X‐ray diffraction (XRD) was conducted to identify crystalline structure. Data were statistically analyzed using two‐way ANOVA and Student's t‐test with a significance level of 0.05. Results The nonaged 3D‐printed 3Y‐TZP group exhibited a significantly higher fracture toughness value (6.07 MPa m 1/2 ) than the milled 3Y‐TZP groups ( p < 0.001). After autoclave aging, the 3D‐printed 3Y‐TZP group maintained significantly higher fracture toughness ( p < 0.001) compared to the milled 3Y‐TZP group. However, no significant differences in hardness values ( p = 0.096) were observed between the aged and nonaged groups within each manufacturing process (3D‐printed and milled) independently. Conclusion The findings revealed that the new 3D‐printed 3Y‐TZP produced by the lithography‐based ceramic manufacturing (LCM) technology exhibited superior fracture toughness after autoclave aging compared to the milled 3Y‐TZP. While no significant differences in hardness were observed between the aged groups, the 3D‐printed material demonstrated greater resistance to fracture, indicating enhanced mechanical stability.