13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- Table 1 Hardness and toughness of the indented samples Sample Load (N) d1 (µm) d2 (µm) d (µm) Vickers hardness (GPa) c (µm) E (GPa) Toughness (MPa/m1/2) #1 10 45.96 47.35 46.66 8.516 71.88 65 0.725 #2 10 41.95 44.35 43.15 9.957 56.60 65 0.960 #3 10 39.92 44.66 42.29 10.367 49.62 65 1.146 #4 50 98.68 70.70 84.69 12.925 178.5 65 0.752 #5 50 98.31 104.5 101.405 9.015 182 65 0.875 #6 50 122.1 125.0 123.55 6.073 194.3 65 0.966 (a) (b) Figure 8 Vickers hardness and toughness determination for specimen Vickers #1 (10N load) (a) measurement of the diagonal length; (b) measurement of the crack length In the results found in the literature, the average toughness of porcelain has been reported to lie in the range 1.5-2.1MPa/m1/2, and the hardness in the range 4-8GPa [7,9,20,21], the results demonstrate that the porcelain used in this experiment has a relatively high hardness, and accordingly a reduced level of toughness. This may be due to the inherent chemical formulation of the porcelain or its processing history. Notably, merely the presence of a population of voids is likely to result in reduced toughness, but may not lead to increased hardness observed in our experiments. 5. Conclusion Conclusions are made based on the ESEM and EDX analyses conducted for this paper. The strength of the zirconia-porcelain interface is derived from the mutual diffusion of zirconium and silicon at the interface. Chipping mode fractures observed in the veneering porcelain indicate the dominance of the cohesive fracture. Adhesive fracture may only be observed in the case of crack travelling at a very shallow angle to the interface. Failure of all-ceramic zirconia-based restorations by porcelain veneer chipping is a complex process that depends on a large number of factors. Pre-existing defects in the porcelain veneer may promote chipping mode cracking and final failure. The results from fractured surface examination of the porcelain zirconia bars further confirm that in the real prosthesis the crack is likely to initiate at the position in lingual side with maximum stress. Moreover, great Vickers hardness and low toughness suggest the weakness of porcelain. Exercising control over these fabrications appears to be a critical requirement for clinical practice.
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