Interfacial toughness of bilayer dental ceramics

Introduction: Microtensile tests are used to test the bond quality of two substrates. However, when the bond strength is relatively high compared with tensile strength of the substrates, the specimens tend to fail cohesively in either substrate. This is caused by a greater probability of large flaws in each substrate because of the greater volume fraction of the substrate than of the interface. However, because the microtensile test has the potential to produce interfacial fracture, it may be useful to calculate interfacial fracture toughness (IFT).

Objective: To test the hypothesis that a controlled-flaw microtensile test can yield mean IFT values that are not significantly different than the fracture toughness of monolithic ceramic control specimens.

Methods: Lithia-disilicate core ceramic (10mm x10mm x10mm) was processed following the manufacturer's recommendations (e.maxPress, Ivoclar Vivadent). Glass veneer (e.maxCeram Ivoclar Vivadent) was applied incrementally to the bonding area and fired in a furnace. The bimaterial blocks were finished and cut with a low-speed diamond saw to obtain specimens with a 1mm x1mm cross-section. Glass veneer bars were prepared in an aluminum mold, fired, and cut to obtain the same dimension specimens. Before testing, 3 overlapping longitudinal cracks were produced along the interface using a Knoop indenter at a load of 4.9 N. Microtensile bars were tested in tension (0.5mm/min). Fracture surfaces were analyzed to determine the site and dimension of the critical flaws. The specimens whose failure origins were not along the interface were discarded. Fracture mechanics was used to calculate IFT.

Results: The apparent toughness of the core-veneer interface and the control group were 0.45±0.15 (n=11) and 0.61±0.17MPa.m^1/2 (n=5), respectively, and are not significantly different (t-test, p=0.06).

Conclusion: The controlled-flaw microtensile test promotes adhesive failure and appears to be a valid method to determine interfacial toughness of bonded ceramics.

Supported by NIH-NIDCR Grant DE06672-24.