Fracture modes in brittle coatings with large interlayer modulus mismatch
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Fracture modes in brittle coatings with large interlayer modulus mismatch Herzl Chaia) Department of Solid Mechanics, Materials and Structures, Faculty of Engineering, Tel Aviv University, Israel 69978
Brian Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
Sataporn Wuttiphan National Metal and Materials Technology Center, Bangkok 10400, Thailand (Received 6 January 1999; accepted 27 May 1999)
Fracture modes in a model glass–polymer coating–substrate system indented with hard spheres are investigated. The large modulus mismatch between the glass and polymer results in distinctive transverse fracture modes within the brittle coating: exaggerated circumferential (C) ring cracks that initiate at the upper coating surface well outside the contact (as opposed to the near-contact Hertzian cone fractures observed in monolithic brittle materials); median–radial (M) cracks that initiate at the lower surface (i.e., at the substrate interface) on median planes containing the contact axis. Bonding between the coating and substrate is sufficiently strong as to preclude delamination in our system. The transparency of the constituent materials usefully enables in situ identification and quantification of the two transverse fracture modes during contact. The morphologies of the cracks and the corresponding critical indentation loads for initiation are measured over a broad range of coating thicknesses (20 m to 5.6 mm), on coatings with like surface flaw states, here ensured by a prebonding abrasion treatment. There is a well-defined, broad intermediate range where the indented coating responds more like a flexing plate than a Hertzian contact, and where the M and C cracks initiate in close correspondence with a simple critical stress criterion, i.e., when the maximum tensile stresses exceed the bulk strength of the (abraded) glass. In this intermediate range the M cracks generally form first—only when the flaws on the lower surface are removed (by etching) do the C cracks form first. Finite element modeling is used to evaluate the critical stresses at crack initiation and the surface locations of the crack origins. Departures from the critical stress condition occur at the extremes of very thick coatings (monolith limit) and very thin coatings (thin-film limit), where stress gradients over the flaw dimension are large. Implications of the results concerning practical coating systems are considered.
I. INTRODUCTION
Hard coatings on soft substrates are of technological importance for the protection they afford underlying substrates from extraneous contacts or impacts. Important examples include cutting tools, thermal barrier coatings, car windscreens, eye glasses, teeth, and dental restorations. In all these cases it is imperative to ensure that the coating–substrate interfaces are well-bonded, to avoid delamination failures. The coatings in such cases nevera)
Work carried out as Guest Scientist at NIST.
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