Fracture Toughness of Zirconia with Nanometer-sized Precracks Fabricated Using Focused-Ion-Beam Milling
Objectives: Zirconia with 3 mol.% yttria has been widely used for dental crowns and bridges due to its excellent mechanical behavior. Fracture toughness is frequently reported as a measure of zirconia’s mechanical performance. However, performing fracture toughness testing on this nanograin material can be a challenge. For reliable results, fracture toughness testing requires the presence of an extremely sharp precrack in the test specimen. Although ASTM standard C 1421 describes multiple crack-forming methods, creating a consistent, uniform precrack is difficult to achieve in nanograin zirconia. The objective of this study was to investigate an alternative method of pre-notching dental zirconia specimens for fracture toughness testing.
Methods: We present a notching method using focused ion beam (FIB) milling to fabricate nanometer-sized cracks in yttria-doped zirconia specimens (3x4x45 mm). The precrack tip is less than 100 nm, which is smaller than the grain size of the specimens (~300 nm), and is consistent throughout the thickness of the specimen.
Results: The FIB-notched specimens show a much reduced average fracture toughness of 5.70 ± 1.16 MPa√m (n=6) compared to 8.99 ± 0.29 MPa√m (n=5) for the specimens without FIB-notches and 10.05 ± 2.22 MPa√m derived from the cracks generated by Vickers indentation tests (n=12). The process of FIB-milling did not appear to create any monoclinic phase prior to fracture toughness testing. Fractures originated at the FIB-notches, and the notch size can be readily identified post-mortem using a microscope. A significant amount of tetragonal-to-monoclinic phase transformation was observed throughout the fracture surfaces.
Conclusions: FIB milling provides an alternative method to fabricate nanometer-sized precracks that are smaller than the grain size of tetragonal zirconia polycrystal. The fracture toughness determined using FIB-notches was ~5.70 MPa√m, significantly smaller than the specimens with prepracks fabricated using saw blades.