13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- interface (that is marked with an arrow). The combination of these observations suggests that the interfacial bonding between zirconia and porcelain can be classified as strong. (a) (b) Figure 2 ESEM image and corresponding elemental X-ray maps of the region of interfacial bonding 3.2 Pre-existing defects The preparation procedure used for porcelain naturally leads to the formation of gas bubbles that persist as defects within the veneering layer. This may increase the likelihood of crack initiation at these positions under the applied mastication load, and may lead to chipping mode cracking in which the fracture propagates by connecting different defects. Fig. 3 illustrates the presence of the pre-existing pores of a range of sizes within prosthesis #1. The SEM image shown was collected from the occlusal-facing fracture surface remaining after chipping. The characteristic semi-spherical smooth dimples represent the remainder after the passage of a crack through pre-existing pores. The presence of both large pores (>20µm) and smaller pores (~10 µm) is noted. Therefore, it appears that some control over the pore size or the distribution of flaws within porcelain veneer after firing would be helpful in reducing the possibility of crack initiation. Since quality control over individual restorations appears to be prohibitively expensive, this would be probably best exercised through control of the thermal schedules and over the veneer shape and thickness. Figure 3 ESEM image of the porcelain fractured surface, showing presence of pre-existing pore defects 3.3 Chipping mode fracture In most cases, the chippings were contained entirely within the porcelain layer and did not reach the interface with the zirconia core, i.e. the cohesive fracture mode was observed. However, it is conceivable that cracks initiated close to or at the veneering surface may propagate across the unit and through the interface to cause the final failure, in which case the adhesive mode of fracture would be observed. A micrograph of chipping mode failure observed is illustrated in Fig. 4(a). The
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