Eur. Phys. J. AP 22, 15–19 (2003) DOI: 10.1051/epjap:2003019 THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS Damaging of a soft substrate by cracks propagation through its hard coating: AFM observations and finite element simulation M. George a , J. Colin, C. Coupeau, and J. Grilh´e Laboratoire de M´etallurgie Physique, Universit´e de Poitiers, UMR 6630 CNRS, SP2MI, boulevard Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France Received: 3 June 2002 / Received in final form: 28 November 2002 / Accepted: 20 December 2002 Published online: 3 April 2003 – EDP Sciences Abstract. Atomic force microscopy observations of the fine structure of cracks have been carried out for nickel thin films on polyimide substrates under tensile test. For strains of the order of 10%, the cracking of the metallic thin film induces notable damaging of the polymeric substrate. Post-mortem observations of the substrate and finite element simulations of the stress field at the cracks head have been performed to characterize this unexpected damaging: the crack propagation at the film/substrate interface and inside the substrate. PACS. 68.60.Bs Mechanical and acoustical properties – 68.37.Ps Atomic force microscopy (AFM) – 62.20.Mk Fatigue, brittleness, fracture, and cracks 1 Introduction Thin films and coating are commonly used in new tech- nologies for the fabrication of optic, magnetic and elec- tronic devices or for protective applications. High stresses when appearing for example during the deposition pro- cess, can lead, in case of tensile stresses, to the formation of cracks and consequently to some alterations of the re- quired properties. A good understanding of the cracking phenomenon is thus a key point in the improvement of the quality of the films. The last ten years, models have been achieved to ex- plain and predict the propagation of cracks in elastic thin films deposited on elastic substrates. Based on the me- chanics of fracture applied to bi-materials, these models consist in the calculation of the stress intensity factors and energy release rate. Several authors [1–3] have char- acterized as a function of the elastic mismatch between the film and the substrate, the propagation of a crack in the film toward the film/substrate interface as well as the steady state propagation of a crack channeling laterally across the film. The case of a crack penetrating into the substrate has also been studied [2]. Models taking into account the possible yielding of the substrate have been proposed [4,5]. This yielding has been shown to improve the channeling of the crack in the film. A review article of Hutchinson and Suo [6] gives a good overview of the dif- ferent results. Xia and Hutchinson [7] have more recently proposed models for simulating various crack patterns of single and multiple cracks. From the experimental point of view, many observa- tions of cracks by optical and electron microscopy have a e-mail: matthieu.george@etu.univ-poitiers.fr been already reported in the literature [8–10]. In-situ scan- ning electron microscopy has been used to characterize the behavior of coatings submitted to tensile testing [11, 12]. The critical applied stress for the crack to propagate has been estimated experimentally as well as the density of cracks as a function of various parameters such as ex- ternal stresses and the thickness of the film [11]. These in-situ experiments have also shown that interfacial rough- ness can further the propagation of cracks and that the final distribution of the cracks depends on the state of the surface [12]. Numerous mechanical behaviours have been already studied for various bi-layers [8–14]. Metallization of polymers used to manufacture microelectronic compo- nents or biologically compatible materials for medical implants [15], leads to the formation of bi-layers with different elastic coefficients where the mechanical be- haviour of the soft polymer substrate must be taken into consideration. In this paper, the case of a soft polyimide substrate coated with a hard nickel thin film is investigated, when external tensile stress is applied step-by-step to the bi- material. In-situ atomic force microscopy (AFM) obser- vations have been used to describe the fine structure of the cracks, their propagation through the interfaces and the unexpected cracking of the polymeric substrates. Fi- nite element simulations have been finally performed to complete these observations. 2 Apparatus and experimental procedure The tensile test apparatus “Microtest” developed by Deben (UK) is particularly suited to in-situ observations.