Competing fracture modes in brittle materials subject to concentrated cyclic loading in liquid environments: Trilayer st
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Yu Zhang Department of Biomaterials and Biomimetics, New York University College of Dentistry, New York, New York 10010
Brian R. Lawna) Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8500 (Received 19 August 2005; accepted 11 November 2005)
A study is made of top-surface cracks induced in brittle trilayers by cyclic indentation with a hard sphere in water. The trilayers consist of an external brittle layer (veneer) fused to an inner stiff and hard ceramic support layer (core), in turn adhesively bonded to a thick compliant base (substrate). These structures are meant to simulate essential aspects of dental crowns, but their applicability extends to a range of engineering coating systems. The study follows on from like studies of brittle monoliths and brittle-plate/soft-substrate bilayers. Competing fracture modes in the outer brittle layer remain the same as before: outer and inner cone cracks and radial cracks, all of which form in the near-contact zone and propagate downward toward the veneer/core interface. Inner cone cracks and radial cracks are especially dangerous because of their relatively steep descent through the outer layer as well as enhanced susceptibility to mechanical fatigue. Experiments are conducted on model glass/alumina/polycarbonate systems, using video cameras to record the fracture evolution in the transparent glass layer in situ during testing. Each fracture mode can lead to failure, depending on the maximum contact load and other variables (plate thickness, sphere radius). The potentially beneficial role of a stiff intervening core is discussed, along with potentially deleterious side effects of residual thermal-expansion-mismatch stresses.
I. INTRODUCTION
Two preceding papers have described the competition between top-surface fracture modes in monolith brittle materials1 and bilayers consisting of a brittle coating on a compliant substrate (bilayer)2 subjected to cyclic contact loading in liquids. The study is now extended to trilayers with a stiff, hard ceramic layer between the external brittle coating and the compliant substrate. Such trilayers are highly relevant to the performance of veneer/ core dental crowns on dentin,3,4 as well as to a variety of other engineering multilayer coating structures, where the intermediate “core” layer is meant to provide support for an otherwise weaker (but functional) outer “veneer”
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0056 512
http://journals.cambridge.org
J. Mater. Res., Vol. 21, No. 2, Feb 2006 Downloaded: 13 Mar 2015
layer.5 In those two preceding papers, three crack modes shown schematically in Fig. 1 were identified: outer cone cracks (O), inner cone cracks (I), and medial–radial cracks (R). Cyclic loading in water enhances these modes, especially the I cracks—by a hydraulic pumping mechanism—and the R cracks—by accumulated quasiplasticity.6,7 As in bilayers, these top-surface fracture modes may be expec
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