Fracture mode transitions in brittle coatings on compliant substrates as a function of thickness
- PDF / 3,043,544 Bytes
- 10 Pages / 612 x 792 pts (letter) Page_size
- 68 Downloads / 207 Views
The fundamentally changing nature of fracture in brittle coatings on compliant substrates with diminishing coating thickness is examined. Attention is focused on cracking induced by concentrated loading with a spherical indenter at the top surface. It is shown that the fracture mode undergoes transitions, from top-surface ring cracking around the contact (“thick-coating” region) to bottom-surface radial cracking at the lower ceramic surface (“intermediate” region) and, finally, back to surface ring cracking (“thin-coating” region). These transitions reflect a progressively changing stress field in the layer structures and highlight the differences in failure mechanism that may be anticipated at the large- and small-scale levels. Simple fracture relations are derived for each mode, expressing critical loads in terms of coating thickness relative to contact or sphere radius, coating strength and coating/substrate modulus mismatch. Data from finite element simulations and contact experiments on model ceramic/polymer bilayer systems are used to validate the basic elements of the analytical relations and to quantify deviations. Implications of the transitional behavior in relation to the strength of brittle coating/film systems are discussed.
I. INTRODUCTION
Brittle coatings are widely used as mechanical and other functional protection for compliant or soft substrates. Such structures are in widespread use, from thick coatings in engineering and biomechanical applications to ultrathin films in advanced devices. They are highly vulnerable to fracture from concentrated loads, specifically to ring cracking outside the contact at the top surface and to radial cracking immediately below the contact at the bottom surface (i.e., coating/substrate interface).1–20 Which of these competing fracture modes is operative in any given material system depends largely on the coating thickness: ring cracks are most evident at the thickness extremes where contact stresses dominate in either the coating (thick coatings, typically > 1 mm) or the substrate (thin films, typically < 1 m); radial cracks are evident at intermediate thicknesses where plate flexure stresses dominate. Coating thickness is therefore a critical scaling factor in the failure properties of brittle layer systems. Questions inevitably arise as to how valid
a)
Guest scientist, on leave from the Department of Solid Mechanics, Materials and Systems, Faculty of Engineering, Tel Aviv University, Tel Aviv, Israel. b) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0219 1752
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 6, Jun 2004 Downloaded: 06 Apr 2015
it is to extrapolate fracture relations from the macroscopic domain of thick coatings into the submicrometer domain of ultrathin films. To address the issue of coating thickness, we analyze here the conditions for fracture in model bilayer systems consisting of brittle ceramic coatings on compliant polymeric substrates, in concentrated loading from a spherical i
Data Loading...
