Sub-10-micrometer toughening and crack tip toughness of dental enamel

In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited RR-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip’s toughness (KI0,KIII0), the crack’s closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine enamel was studied using atomic force microscopy (AFM). The mode I crack tip toughness KI0 of cracks along enamel rod boundaries and across enamel rods exhibit a similar range of values: KI0,Ir=0.5–1.6MPa m0.5 (based on Irwin’s ‘near-field’ solution) and KI0,cz=0.8–1.5MPa m0.5 (based on the cohesive zone solution of the Dugdale–Muskhelishvili (DM) crack model). The mode III crack tip toughness KIII0,Ir was computed as 0.02–0.15MPa m0.5. The crack-closure stress at the crack tip was computed as 163–770 MPa with a cohesive zone length and width 1.6–10.1μm and 24–44 nm utilizing the cohesive zone solution. Toughening elements were observed under AFM and SEM: crack bridging due to protein ligament and hydroxyapatite fibres (micro- and nanometer scale) as well as microcracks were identified.