Sliding contact damage in thin films with low adhesion

Surface functionalization of flat glass (optical, thermal, electrochemical ...) by the deposition of thin film by magnetron sputtering is a widely used technique. For example, low emissivity glasses allow reducing energy losses by blocking infra-red while keeping the glass transparent. Those functionalized glasses are based on multi-layered thin films that contain a silver layer (ten nanometres). However, these coatings have often low mechanical resistance toward contact loading (scratch) because of their low adhesion. However, a quantitative analyse of the damage produced by sliding contact is not trivial because of the complexity and the thinness of the layers (few nanometres) and also because these damage strongly depend on mechanical properties of the stack (modulus, toughness, adhesion …) [1]. The idea developed during this study is to extend the understanding of the scratch phenomenon to better understand the crucial role of adhesion by using patterned samples. Indeed, it has been recently showed that an alternation of weak adhesion areas and strong adhesion areas allows improving adhesion [2, 3].

In this study, the objective is to take advantage of the transparency of our samples to perform direct visualization of the damage during sphere on plane tests carried out on thin multilayer coatings deposited on glass. First of all, we focus on homogenous samples with a weak interface and we show that the scratch results from the interaction between the debris that are produced and the coating. Then, we use different patterning protocol (photolithography, masking, self-patterning…) to modify and control the adhesion of our multilayers. We analyse the transition between strong and weak interfaces to understand the influence of adhesion on scratch mechanisms and characteristics thanks to the analysis of the evolution of many parameters such as the distance between the surface and the sphere, the generation of debris, the initiation morphology or the width of the scratch.

References
[1] X. Geng, Z. Zhang, E. Barthel, D. Dalmas, Mechanical stability under sliding contact of thin silver film embedded in brittle multilayer, Wea,r 276– 277, pp 111– 120, (2012)
[2] D. Dalmas, E. Barthel, D. Vandembroucq, "Crack front pinning by design in planar heterogeneous interface", J. Mech. Phys. Solids 57, pp 446, (2009)
[3] Patinet, S., Alzate, L., Barthel, E., et al., Finite size effects on crack front pinning at heterogeneous planar interfaces: Experimental, finite elements and perturbation approaches, J. Mech. Phys. Solids, 61, Issue: 2, pp 311-324, (2013)