Materials Science and Engineering A 419 (2006) 189–195 Effect of coating thickness on crack initiation and propagation in non-planar bi-layers Tarek Qasim, Chris Ford ∗ , Malika Bongu´ e-Boma, Mark B. Bush, Xiao-Zhi Hu School of Mechanical Engineering, The University of Western Australia, 35 Stirling Highway Crawley, WA 6009, Australia Received in revised form 15 December 2005; accepted 15 December 2005 Abstract Hertzian contact damage is studied in glass coatings (thickness range 160 m to 1 mm) on polycarbonate polymer substrates. Both planar and non-planar geometries are considered, subjected to indentation by fixed size spherical indenters of radius 4 mm. Finite element analysis is carried out to evaluate the stress distribution in the bilayer structure. Radial cracking initiating at the coating undersurface directly under the indenter is the primary focus of this investigation, and cone cracking at the top surface of the coating (inner and outer cone cracks) is also considered. It is concluded that crack propagation is facilitated in coatings of an intermediate thickness. Thick (1000 m) coatings resist deflection, decreasing tensile stresses at the coating undersurface, while thin (160 m) coatings deflect massively, causing a compression zone beneath the indenter which also limits undersurface tension. © 2006 Published by Elsevier B.V. Keywords: Finite element analysis; Indentation; Crack; Shape irregularity; Bilayer 1. Introduction The development of systems combining the wear resistance of a brittle coating and the toughness of a ductile under layer is very important for high damage tolerance applications. The mechanical response of brittle layered structures under Hertzian indentation is of considerable interest in disciplines such as biomechanics and tribology, and in the field of dentistry, for tooth restorations such as crowns [1,2]. The contact damage tolerance and failure resistance of such systems is affected by a variety of parameters. Careful selection of component properties (stiff coating/soft interlayer) and coating geometry (planar/non- planar) may therefore be used to maximize the damage resistance of such systems. A considerable amount of experimental and analytical inves- tigation of systems utilising materials relevant to dental prosthe- ses has been carried out using Hertzian indentation, including work by Zhao et al. [3–5], Lee and Lawn [6,7], Chai [8], Ford et al. [9] and Shrotriya et al. [10]. However, the bulk of the pre- ∗ Corresponding author. Tel.: +61 8 6488 1901; fax: +61 8 6488 1024. E-mail address: chrisf@mech.uwa.edu.au (C. Ford). vious work focused on the response of flat bilayer or trilayer systems, which did not account for the effects of coating curva- ture. The principle modes of failure observed in these studies are shown in Fig. 1: plastic yielding of the substrate, cone cracking (Hertzian and outer cracks) at the upper surface of the coating, radial (interface) cracks at the lower surface of the coating. Crit- ical loads for radial cracks and cone cracks are denoted as P r and P c , respectively. Different failure modes dominate for different material combinations and geometries. Previous studies by the authors [9,11–13] examined the effects of changing curvature, of both the indenter and the indented samples. Experimental studies [11–13] were inter- preted using finite element methods [12,13] and concluded that curved surfaces on compliant substrates are generally more resistant to initiation of the dominant radial cracking mode than equivalent planar systems, although convex curvature may enhance subsequent crack growth to failure [13]. This paper considers the effect of coating thickness, which while well documented in flat multilayers, has not previously been closely examined in curved systems. The focus of this study is the radial crack system, identified as the primary failure mode in brittle coatings on compliant substrates [14–16], and mention is also made of cone cracking (usually a secondary mode for this 0921-5093/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.msea.2005.12.023