Kimheng MENG – Damage mechanism by coating blister: Influence of plasticity and delamination

Jury

Vincent GUIPONT (Charge de recherche, Mines Paris Tech), Reviewer
Cécile LANGLADE (Professeure des universités, Université de Technologie de Belfort-Montbéliard), Reviewer
Megan CORDILL (Senior scientist, Erich Schmid Institute for Materials Science), Examiner
Jean-Yvon FAOU (Ingénieur de recherche, Saint-Gobain Recherche), Examiner
Marc VERDIER (Directeur de recherche, CNRS Alpes), Examiner
Damien FAURIE (Professeur des universités, Sorbonne Université Paris Nord), Examiner

Abstract

Thin films and coatings, used in a wide range of technological applications, are often industrially produced by physical vapor sputtering techniques, which may lead to very high levels of internal stresses. These high stresses trigger damage mechanisms, such as buckling-driven delamination. Preventing buckling is hence a major issue for industrial applications of coatings. Various buckling morphologies have been commonly observed, such as straight-sided buckles (SSB) and circular blisters (CB). Those buckle morphologies often evolve into more complex buckles by a combined effect of secondary buckling and crack front propagation, giving rise to structures known as telephone cords or varicose structures.

In this work, we report on straight-sided buckles and circular blisters experimentally observed by optical and atomic force microscopy on gold ductile thin films deposited by physical vapor deposition on silicon wafers. It is shown that, whatever the buckle dimensions, their maximum deflections are higher than those expected by the elastic theory (based on the Föppl-Von Karman plate equations). By using finite element method (FEM) simulations, it is shown that taking into account plastic deformations in the film well explains the buckling structures experimentally observed. The elastic moduli of the thin film, as well as the full set of parameters of the plasticity hardening law, have been identified using nanoindentation tests coupled with finite element solutions. The numerical results are presented, discussed, and compared to the experimental observations, shedding some light on the genesis of those structures, highlighting in particular the importance of taking the film loading history (buildup of stress inside the film during deposition) into account. In addition, we investigate the growth of circular blisters from initial debonding defects. For elastic thin films, this growth is known to be unstable, leading to non-axisymmetric shapes. This result is confirmed by our FEM calculations, coupling buckling of a plate with a cohesive zone model (CZM). In addition, a novel result has been obtained: plastic deformation plays a key role in stabilizing the circular shape of the blisters during growth. This explains why circular buckles can be observed on gold films.