Cracking of brittle coatings adhesively bonded to substrates of unlike modulus
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Herzl Chai Department of Solid Mechanics, Materials and Structures, Faculty of Engineering, Tel Aviv University, Israel 69978 (Received 10 March 2000; accepted 22 May 2000)
The role of elastic mismatch in determining critical conditions for indentation fracture in brittle coatings on substrates of unlike modulus was investigated. A model transparent trilayer system, consisting of a glass coating layer bonded to a thick substrate of different glass or polymer by a thin layer of epoxy adhesive, facilitated in situ observations of crack initiation and propagation. A tungsten carbide sphere was used to load the layer system. Abrasion flaws were introduced into the top and bottom glass coating surfaces to control the flaw populations and to predetermine the origins of fracture: cone cracks occurred at abraded top surfaces, radial cracks at abraded bottom surfaces. Analytical relations for the critical loads are presented for each crack system in terms of elastic modulus mismatch, indenter and coating dimensions, and material fracture parameters. Implications concerning materials selection for resistance to crack initiation are considered.
The problem of a brittle coating bonded to a substrate is of practical importance in layer applications. Engineering structures such as cutting tools, laminated windows and thermal barrier coatings, and natural structures like shells, teeth, and artificial crowns are examples. Hard outer layers provide resistance against damage from external influences (predominantly mechanical but also thermal and chemical); soft inner layers provide stress redistribution and crack containment. The adhesive needs to be sufficiently well bonded to the adjacent brittle layers to preclude delamination failures and soft enough to prevent sharp transverse cracks from penetrating into adjacent layers. Such considerations are especially important in the case of cracks formed from contacts or impact at the outer coating surface. Unfortunately, a soft support also allows the coating to flex beneath the contact, leading to changes in coating fracture mode from surface cone crack to a more dangerous subsurface radial crack. Such transitions in crack mode have been well documented for brittle coatings fused to soft substrates.1–13 In situ observations in a model transparent bilayer system consisting
a)
Guest Scientist from Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yusong, Taejon 305-701, Korea. b) Ceramics Division. J. Mater. Res., Vol. 15, No. 8, Aug 2000
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of a soda-lime glass layer bonded to a plastic substrate have proved especially useful in describing such modes.14 Recently, we have examined a trilayer system in which an upper soda-lime glass layer (coating) is bonded to a like lower glass layer (substrate) with a soft epoxy adhesive.15 In that latter case it is the soft adhesive rather than the substrate that allows the flexure mode to develop and generate radial fractures. Here we extend t
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