Fracture of Brittle Layers Joined with High Elastic Modulus Composite

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Ceramic properties such as biocompatibility and inertness have secured their use in biomedical prosthetics. The brittle nature of ceramics governs their application in any design and fabrication technique. Current all-ceramic dental crowns have a reported failure rate of approximately 3% a year. An investigation into a possible improved design over current all-ceramic dental crowns is performed. Current methods of fabricating all-ceramic dental crowns involve laborious and time consuming application of porcelain veneer layers onto a core material. The proposed design is to join independently fabricated veneer and core layers together using a high elastic modulus composite.

Fracture behavior of brittle layers joined by a high elastic modulus composite is studied in this dissertation. There are two dominant fracture mechanisms of concern for dental crowns when joining brittle layers with a more compliant interlayer; the formation of radial cracks in the veneer or core and the propagation of cracks between brittle layers. The occlusal loading on dental crowns can be simulated with the use of Hertzian contact testing on flat brittle laminates, which allow for the study of radial cracks in the veneer. It is shown for the first time that bottom-surface radial cracks in the veneer due to flexure can be suppressed using a high elastic modulus joining interlayer. The relationship between the critical loads for radial crack formation, Pcr and the interlayer modulus and thickness is elucidated. Furthermore, using the high modulus composite as an interlayer increases the long term cyclic loading lifetime over joins with similar moduli to currently available dental adhesives. The propagation of cracks between adjacent brittle layers is shown to be controlled by a reinitiation mechanism and not penetration through the adhesive. Cracks that traverse the layer of origin arrest at the join interface in brittle laminates. Reinitiation loads are dictated by strength of the adjacent brittle layer and modulus of adhesive. This study shows that it is possible to use a high modulus composite as a joining material in the fabrication of dental crowns, while suppressing the formation of radial cracks in the veneer and limiting the propagation of cracks between adjacent brittle layers.