Contact fracture of brittle bilayer coatings on soft substrates
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nia Pajares Departamento de Fı´sica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain
Fernando Guiberteau Departamento Electro´nica e Ingenierı´a Electromeca´nica, Escuela de Ingenierı´as Industriales, Universidad de Extremadura, 06071 Badajoz, Spain
Francisco L. Cumbrera Departamento de Fı´sica, Facultad de Ciencias, Universidad de Extremadura, 06071 Badajoz, Spain
Brian R. Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 22 May 2000; accepted 21 September 2000).
Contact-induced fracture modes in trilayers consisting of a brittle bilayer coating on a soft substrate were investigated. Experiments were performed on model transparent glass/sapphire/polycarbonate structures bonded with epoxy adhesive, to enable in situ observation during the contact. Individual layer surfaces were preferentially abraded to introduce uniform flaw states and so allowed each crack type to be studied separately and controllably. Fracture occurred by cone cracking at the glass top surface or by radial cracking at the glass or sapphire bottom surfaces. Critical loads for each crack type were measured, for fixed glass thickness and several specified sapphire thicknesses. Finite element modeling (FEM) was used to evaluate the critical load data for radial cracking, using as essential input material parameters evaluated from characterization tests on constituent materials and supplemental glass/polymer and sapphire/polymer bilayer structures. The FEM calculations demonstrated pronounced stress transfer from the applied contact to the underlying sapphire layer, explaining a tendency for preferred fracture of this relatively stiff component. Factors affecting the design of optimal trilayer structures for maximum fracture resistance of practical layer systems were considered.
I. INTRODUCTION
Hard ceramic coatings on soft substrates are of technological importance for the protection they afford substrates against contacts, impacts, and thermal or chemical cycling. Practical examples of layer systems with ceramic coatings include cutting tools,1 thermal barrier coatings,2,3 and dental crowns.4,5 An ideally hard coating protects the soft underlayer by maintaining the entire system in the elastic region. But hard coatings tend to be brittle and are subject to failure from transverse fracture or other damage modes, especially in contact loading where stress intensities are highly concentrated.6 –16 The fracture modes include traditional cone or ring cracks that initiate from the top surface outside the contact area and deleterious radial cracks that initiate at the lower surface of the coating and spread radially outward on J. Mater. Res., Vol. 16, No. 1, Jan 2001
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median planes containing the load axis. Recent studies of model transparent glass/polymer bilayers have proved especially useful in evaluating such cracking modes in situ during contact loading and for quantifying the crac
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