Coatings Adhesion PRopertIes measurement and Control – CAPRICe

Buckling delamination (experimental): Couples of film / substrate materials of interest are defined, with a strong elastic «contrast« between the two materials. Thin films are produced by Physical Vapor Deposition so as to induce high internal stresses. The thicknesses of deposited films are characterized (by confocal optical microscopy). The internal stresses within these films are estimated. The blistering structures are generated within the films by uniaxial compression of the elastic substrates, their morphologies characterized by AFM and their propagation controlled in order to cause them to interact, or even coalesce.

Buckling delamination (modeling / numerical computation): A delamination study is carried out, integrating the coupled buckling and delamination aspects as well as the elasticity of the substrate. It is of high interest to see how straight-line or cord-like blisters propagation spreads in this context. The case of two wrinkles in interaction via the deformable substrate is particularly interesting to study.

Multi-cracking (experimental): In the case of ZnO monolayers, the study of the fragmentation showed a saturation regime leading to a micrometer size strips structure for thin layers. We want to understand the phenomena through a comparison with simulations. In particular, the influence of the plastic dissipation in the flexible substrate on the cracking structure remains an open question. Uniaxial tensile tests on ZnO / ETFE samples and ZnO / Ag / ZnO / ETFE stacks are carried out, the cracks structure being followed by optical microscopy.

Multi-cracking (modeling / numerical calculation): 2D plane strain calculations are carried out, including plasticity in the substrate, fracture within the film, but also delamination at the film / substrate interface. Crack nucleation / growth is managed by cohesive zone models.

Buckling delamination:
Calculations of buckling delamination couping a geometric nonlinear plate model and a cohesive zone model allowed to explain and quantify the «sombrero« shape of the blisters in the case where they lay on a flexible substrate. Moreover, it has been possible to demonstrate that the propagation mechanism is quite different from what could be expected due to this significant depression at the crack front. These results have been published in an international journal (referred below), as well as papers at conferences.

Multi-cracking:
Original modeling of the multi-cracking could be carried out by introducing a Gaussian distribution of toughness within the film, representing the distribution of defects in the oxide layer. The relationship between the plasticity of the substrate and the saturation of the inter-cracks distance in strongly deformed regimes has been established. The publication of an article in an international journal synthesizing these results is planned. These results were shown in communications at conferences.

Buckling delamination:
Current studies focus on the interaction of straight-sided blisters and telephone cords blisters propagation on a deformable substrate. Under what conditions do they interact? Does the elasticity of the support promote a particular change in morphology as compared to the case of the rigid substrate? A publication is being written on this subject.

Multi-cracking:
A first outline concerns the presence of a complex stack of the ZnO / Ag / ZnO type, inside which delamination and plasticity can occur in addition to the plasticity of the substrate and the delamination between the substrate and the stack. In particular, the effect of the dissipation in the intermediate layer of silver is interesting.
A second outline relates to cracking in the 3D case, with the issue of straight cracks versus undulated cracks transition.

Articles published in international journals:

How soft substrates affect the buckling delamination of thin films through crack front sink-in, R. Boijoux, G. Parry, J.-Y. Faou, and C. Coupeau,Appl. Phys. Lett. 110, 141602 (2017)

Communications in international conferences

I. Ben Cheikh, G. Parry, R. Estevez, D. Dalmas, Study of multi-cracking of brittle thin films and brittle/ductile multilayers on compliant substrate,ECF21, Catania, Italy, June 20-24 2016.
I. Ben Cheikh, G. Parry, R. Estevez, D. Dalmas, J. Teisseire Study of the multi cracking of brittle films and assemblies of brittle/ductile films bonded on compliant substrate, Euromech Colloquium 570, Houffalize, Belgium,20 October – 23 October 2015.
R. Boijoux, G. Parry, C. Coupeau, Influence of the substrate elasticity on interactions and coalescence of thin films bucking structures, Euromech Colloquium 570, Houffalize, Belgium,20 October – 23 October 2015.

Nowadays thin films are used in everyday life. In particular, sputter-deposited coatings are crucial in industrial applications. However, high residual stresses may lead to disruption of the coating and loss of its functional properties. Such disruption is intimately related to two mechanical phenomena: fracture and buckling. (Numerical modeling of such phenomena is extremely challenging and rarely performed properly.) Recent advances in understanding the buckling phenomenon on hard substrates (the relation between blister’s morphology and the interfacial toughness) have been reached by the participants of the current project through advanced numerical modeling and controlled experimental observations. Such strategy can be extended to another framework: plastic (fragile or ductile) films on a soft substrate, a major system in fast growing field of flexible electronics. In this project we will again combine the experimental and numerical approaches in a study of thin films (from a few tens to a few hundreds nm) on flexible substrates (polymers) allowing for controllable deformation (tension, compression and bending). We will study composite stacks consisting of metallic and oxide layers, which might exhibit both ductile and brittle behavior depending on geometry. The system will be studied both in compression and in tension. In compression, blistering of the film is expected. In tension, the deformation may reach tens of percent and will result in film cracking or other plastic deformation. For both cases, numerical modeling will account for the contrast in elastic properties as well as the mechanisms of plastic deformation. With such approach we aim to identify the key parameters governing the mechanical instability of the system.