Crack front pinning at patterned planar interfaces in multilayer thin films

Thin film multilayers deposited on glass are widely used for flat optical, photoelectric and electrochromic devices. In many applications, adhesion is a crucial issue. For instance, adhesion of thin film multilayers deposited on glass is a crucial issue for many industrial applications. Thus, it becomes of great interest to increase the adhesion. Many mechanisms of toughening a brittle solid can be found in literature (local transformations, crack bridging, crack trapping, microcracking …); but few of them can be applied to thin film layer. One possible way to increase the adhesion of an interface between two layers is to create a composite interface in order to modify the crack front morphology due to pinning on region of higher toughness [1]. In this study, we show that it is possible to understand and also to improve the adhesion of thin multi-layer coating by a method of interfacial patterning. A previous work showed that patterning the silver layer by direct masking during deposition allow to alternate weak adhesion areas (with silver) and strong adhesion areas (without silver) [2] (figure 1(a)). This pattern allows the global adhesion to be improved (~ toughening) due to pinning of the crack in the area of higher adhesion [2, 3]. In order to test more complex textures and to further improve toughening, a new protocol based on a photolithography method has been proposed. In contrary to direct masking, this protocol allows to test many different and complex textures. Cleavage tests were performed on model multi-layer patterned coatings. As the transparency of glass allows a direct visualization of the crack front’s morphology, it is possible to validate this new patterning method by analyzing this morphology within the framework of crack pinning theories [1, 3] (figure 1 (b)). References: [1] H. Gao and J.R. Rice, First-Order perturbation analysis of crack trapping by arrays of obstacles. JMPS, 56 (1989) 828-826 [2] D. Dalmas, E. Barthel, D. Vandembroucq, "Crack front pinning by design in planar heterogeneous interface", J. MECH. PHYS. SOLIDS 57 (2009) 446 [3] S. Patinet, L. Alzate, E. Barthel, et al., Finite size effects on crack front pinning at heterogeneous planar interfaces: Experimental, finite elements and perturbation approaches, JMPS, 61, Issue: 2 (2013) 311-324