13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- verified by the detailed analysis of fractographic features shown in Figs. 7(b) and 7(c) (images of one representative sample) that indicate the crack propagation from “top” to “bottom” (marked with white dashed arrow Fig.7(b)), although the features were only visible on the “left” and “right” sides, or both sides of the cross section (marked with red circle in Fig. 7(a)) due to the breakaway of the porcelain on the “top” or “bottom” sides. Figure 7 Schematic representation of the three-point bending test and failure of the specimens (a) three-point bending; (b) & (c) typical ESEM images of the fractured surfaces 4.2 Vickers hardness and toughness evaluation Further evaluation of the mechanical properties of the veneering porcelain (hardness and fracture toughness) was carried out using Vickers indentation on six zirconia porcelain bar samples (designated “Vickers #1-#6”). The formulas used for the evaluation of Vickers hardness and toughness are [8,20,21]. 2 1.854 F F H A d = = (1) 0.5 1.5 0.016 IC E F K H c ⎛ ⎞ ⎛ ⎞ = ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠ (2) where F is the applied force on the diamond indenter (unit of N), A is the surface area of the resulting indentation (unit mm2), d the average length of the diagonal left by the indenter (the average of two diagonals d1 and d2) with the unit of mm, c the crack size measured from the center of the indentation (unit of mm), H is Vickers hardness (unit of GPa) and E is Young’s modulus (unit of GPa), and IC K is the fracture toughness (unit of MPa/m1/2). For illustration, the indentation impression in sample Vickers #1 is shown in Fig. 8. The results of all six indented samples with loads of 10N and 50N are listed in Table 1.
RkJQdWJsaXNoZXIy MjM0NDE=