Crack initiation modes in bilayered alumina/porcelain disks as a function of core/veneer thickness ratio and supporting substrate stiffness

We hypothesize that the fracture resistance of alumina core/porcelain venee r disks increases and that crack initiation shifts from veneer to core as t he core/veneer thickness ratio (t(C)/t(V)) increases from 0.5/1.0 to 1.3/0. 2, or as the elastic modulus of the supporting substrate (E-S) to which it is resin-bonded increases from 5.1 to 226 GPa. When supported by a low-modu lus substrate, disks with low t(C)/t(V) ratios exhibited cracks in the vene er and within the core, while those with high t(C)/t(V) ratios demonstrated core cracks, but not veneer cracks. None of the disks supported by Ni-Cr a lloy (E = 226 GPa) exhibited core cracks. These results support the hypothe sis that the crack initiation site shifts as the t(C)/t(V) ratio increases, but the increase in E-S did not affect the crack initiation site. This stu dy suggests that the t(C)/t(V) ratio is the dominant factor that controls t he failure initiation site in bilayered ceramic disks.